Lecture Notes

ARKA JAIN University, Jharkhand

Department of EEE SEC C

Electric Power Transmission & Distribution

COURSE CODE- DIP14079

Diploma EEE(4th Semester)

Name of the Faculty: Prof. Taniya Ghosh

Designation: ASST. PROFESSOR (EEE)

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Module1
BASICS OF TRANSMISSION AND DISTRIBUTION: Single line diagrams with components of the
electric supply transmission and distribution systems. Classification of transmission lines: Primary
and secondary transmission; standard voltage level used in India. Classification of transmission
lines: based on type of voltage, voltage level, length and others Characteristics of high voltage for
power transmission.
Method of construction of electric supply transmission system 110 kV, 220 kV, 400 kV. Method of
construction of electric supply distribution systems 220 V, 400V, 11 kV, 33 kV

Single line diagrams with components of the electric supply transmission and distribution systems

A single line diagram for an electric supply transmission and distribution

system would typically include: power generation source (generator),
transmission lines, step-up transformers at the substation, high voltage
transmission lines, step-down transformers at distribution substations,
distribution lines, and finally, individual loads connected to the low voltage
distribution lines; all depicted using standardized symbols to represent
each component, with a single line representing all three phases of the AC
power system.
Key components shown on a single line diagram:
 Generation Source:
Represented by a generator symbol, often with a power rating
indicated.
 Step-up Transformer:
A transformer symbol located at the generating station to increase
voltage for transmission.
 Transmission Lines:
Single lines representing the high voltage transmission lines connecting
generating stations to substations.
 Transmission Substation:
Shown as a busbar with circuit breakers, lightning arrestors, and other
protective devices.
 Step-down Transformer:
Transformer symbol at the distribution substation to reduce voltage for
distribution.

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 Distribution Lines:
Lines depicting the lower voltage lines feeding power to smaller
substations or directly to consumers.
 Distribution Substation:
A smaller substation with transformers and feeder circuits for
distributing power to local areas.
 Loads:
Symbols representing individual or aggregated consumer loads like
homes, industries, or commercial buildings.
Important points to remember about single line diagrams:
 Simplification:
They only show one line to represent all three phases of a power
system, making the diagram easier to read and analyze.
 Standard Symbols:
Each component is represented by a standardized symbol, which
allows for easy interpretation.
 Protection Devices:
Circuit breakers, fuses, and lightning arrestors are typically included on
the diagram to show protection measures.
 Voltage Levels:
Different voltage levels within the system are usually indicated on the
diagram.

Electrical energy, after being produced at generating stations (TPS, HPS, NPS, etc.)
is transmitted to the consumers for utilization. This is due to the fact that generating
stations are usually situated away from the load centers. The network that transmits
and delivers power from the producers to the consumers is called the transmission
system. This energy can be transmitted in AC or DC form. Traditionally, AC has
been used for years now, but HVDC (High Voltage DC) is rapidly gaining popularity.

Single line diagram of AC power transmission system

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What are the standard voltage levels used in India for transmission lines?
In India, the generation voltage levels are 11 kV and 33 kV. But for Power
transmission, 220 kV, 400 kV and 765 kV are used. The maximum voltage at which
AC power is transmitted in India is 765 kV.

Classification of transmission lines: based on type of voltage, voltage level, length

Transmission lines are primarily classified based on their length as "short,"

"medium," and "long" lines, with each category corresponding to a different
voltage level, where short lines have lower voltages, medium lines have
moderate voltages, and long lines have very high voltages; typically, short
lines are less than 80km, medium lines range from 80-250km, and long
lines exceed 250km.
Based on Length:
 Short Transmission Line:
Length less than 80km, voltage usually below 20kV, where line
resistance and inductance are dominant factors.
 Medium Transmission Line:
Length between 80-250km, voltage ranging from 20kV to 100kV,
requiring consideration of all three line constants (resistance,
inductance, and capacitance) as "lumped" elements.
 Long Transmission Line:
Length exceeding 250km, voltage above 100kV, where line constants
are treated as distributed elements due to significant wave propagation
effects.
Based on Voltage Level:
 Low Voltage (LV): Typically below 1kV, used for distribution within
buildings.
 Medium Voltage (MV): Between 1kV and 69kV, often used for
distribution networks.
 High Voltage (HV): Between 69kV and 230kV, used for longer
transmission distances.

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  Extra High Voltage (EHV): Above 230kV, used for very long
distance power transmission.
 Ultra High Voltage (UHV): Above 800kV, considered for extremely
long transmission needs.

The Types of Transmission Lines Based on Voltage

Key Takeaways

 According to environmental conditions, geography, sensitivity, and cost,

transmission lines are either made overhead or underground.
 The allowable voltage range in medium transmission lines ranges from 20 to 100
kV.
 Low tension cables are used for voltages up to 1kV in underground transmission
line systems.

Transmission lines are the connectors running between generating stations and distribution
stations. Transmission lines carry high voltages from the generating stations to primary
transmission stations, secondary transmission stations, primary distribution stations, and
secondary distribution stations.

These lines are classified based on their location (overhead or underground), length, and
voltage rating. Among these three characteristics, understanding how the classification of
different types of transmission lines based on voltage works is particularly important for
choosing the right cable for a given voltage level. Outside of characteristics
like power of distribution lines and transmission cables, from a design perspective there are
also features like characteristic impedance, propagation delay, induction, and reflected waves
among other transmission line effects to track.

Let’s explore two classifying characteristics, the location of the line and the voltage rating, and
see how they relate to each other.

Overhead and Underground Transmission Lines

Transmission lines can either be located overhead or underground.

Overhead transmission lines are bare conductors above the ground level, supported by pylons
and poles. The major parameter classifying overhead transmission lines is their length. For each
length classification of overhead cables, there is a maximum voltage limit beyond which they are
not permitted.

Underground transmission lines are insulated cables that are buried under the ground inside
vaults and trenches. Voltage levels and insulation classify underground cables. There is a
specific type of underground cable for each class of voltages.

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When deciding on whether a transmission line should be overhead or underground,
environmental conditions, geography, the line’s sensitivity, and costs should be taken into
consideration.

Types of Transmission Lines Based on Voltage

Both overhead and underground transmission lines have subclassifications based on voltages.

Overhead Transmission Lines

a. Short transmission lines - In short transmission lines, the length is within 50km
and the voltage is limited to less than 20 kV. In short transmission lines, the
effect of line resistance and inductance is more predominant than capacitance.
b. Medium transmission lines - These lines have an overhead cable length of
greater than 50km and less than 150km. The allowable voltage ranges from 20
to 100 kV. The analysis of medium transmission lines considers the three
lumped line constants: resistance, inductance, and capacitance.
c. Long transmission lines - Overhead transmission lines with lengths greater
than 150km and voltages above 100kV form long transmission lines. Line
constants are considered distributed elements in the analysis of long
transmission lines.

Underground Transmission Lines

Unlike overhead cables, underground cables consist of one or more conductors with insulation
and protective covering over it. The basic construction of underground transmission lines
consists of parts such as the core or conductors, insulation, metallic sheath, bedding, armoring,
serving, etc.

There are several types of underground cables available on the market. Selecting the
appropriate underground cable involves considering the working voltage and service
requirements.

The classification of underground cables is done in two ways:

1. Classifying based on the voltage for which the underground cables are
manufactured.

2. Classifying based on the insulation used in the cable’s construction.

The table below gives the classification of underground cables based on voltages.

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 Classification of underground cables based on voltage

Depending on the geographical area, environmental conditions, service requirements, and cost,
designers can make the appropriate choice between overhead and underground cables. A good
understanding of the types of transmission lines based on voltage will ease the tasks of cable
selection, installation, maintenance, and repair.

Whether you're looking at general electricity or energy generation, renewable energy, or just
trying to better optimize power line and overhead line behavior in your
next transmission project, make sure you have your bases covered. Make sure your wires aren't
crossed, especially when they are high voltage, and avoid common design struggles
like attenuation, capacitance, and characteristic impedance.

Based on Configuration
Overhead Transmission Lines: Overhead lines are the most common type, with conductors
suspended from towers or poles above the ground. They are cost-effective, easy to construct,
and maintain.
Underground Transmission Lines: These lines have conductors buried underground, which
makes them less visible and reduces the impact on landscapes. However, they are more
expensive to install and maintain compared to overhead lines.
Submarine Transmission Cables: Submarine cables are used for power transmission across
bodies of water, such as oceans or large lakes. They are essential for interconnecting islands
or crossing water barriers.
Based on Conductor Types
ACSR (Aluminum Conductor Steel Reinforced): These conductors consist of a central steel
core surrounded by aluminum strands. ACSR conductors offer both strength and conductivity,
making them widely used in overhead transmission lines.
AAC (All Aluminum Conductor): AAC conductors consist of only aluminum strands and are
lighter than ACSR conductors. They are commonly used in moderate voltage applications.
ACCC (Aluminum Conductor Composite Core): ACCC conductors have carbon and glass
fibers in the core, providing higher strength and thermal capacity compared to traditional
conductors.

others Characteristics of high voltage for power transmission.

High voltages are more suitable than low voltages for the transmission of electrical energy because
loss of energy due to conductor resistance is less with high voltages. However, the voltage cannot be
increased indefinitely

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Key characteristics of high voltage power transmission include: reduced energy loss due to lower
current at higher voltage, allowing for thinner conductors and lighter transmission towers, improved
efficiency over long distances, the need for specialized equipment like transformers to step up and
down voltage levels, increased potential for corona discharge at very high voltages, and the ability to
transmit large amounts of power with smaller conductor sizes; making it the preferred method for
long-distance electricity transport.
Breakdown of characteristics:
 Lower current, less loss:
When voltage is high, the current required to transmit the same amount of power is lower,
resulting in significantly reduced resistive losses in the transmission lines due to the formula P =
VI (power = voltage x current).
 Thinner conductors:
Because of the lower current, smaller diameter conductors can be used, which reduces the
weight of the transmission lines and the cost of materials.
 Long-distance efficiency:
High voltage transmission is particularly advantageous for transmitting electricity over long
distances, minimizing energy losses during transport.
 Transformer requirement:
To utilize high voltage in homes and industries, transformers are needed to step down the
voltage to a usable level at the receiving end.
 Corona discharge:
At very high voltages, corona discharge can occur around conductors, which is a loss
mechanism where energy is dissipated into the surrounding air.
 Complex design considerations:
High voltage transmission lines require specialized design features, including large insulators
and robust towers to manage the high voltage and prevent arcing.

Method of construction of electric supply transmission system 110 kV, 220 kV, 400 kV.

The construction of an electric supply transmission system for 110 kV, 220 kV, or 400 kV
involves land work, preparing foundations, and installing conductors. The configuration of the
towers is determined by the required clearances, the height of the tower, and the size and
material of the conductors.
The construction of an electric supply transmission system at 110kV, 220kV, and 400kV
involves several key steps: route selection, tower foundation construction, tower erection,
conductor stringing, insulator installation, grounding, and substation construction, with each
voltage level requiring specific design considerations for tower structure, conductor size, and
clearance distances due to increased voltage levels.
Key Stages:
1. Pre-Construction Phase:
 Route Survey and Selection:
 Conduct detailed surveys to identify the most suitable route considering factors
like land ownership, terrain, environmental impact, population density, and
existing infrastructure.
 Obtain necessary permits and approvals from relevant authorities.
 Design Engineering:

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  Determine tower type and spacing based on voltage level, terrain, and load
requirements.
 Select conductor size and material considering power transmission capacity and
line losses.
 Design substation layout including transformers, switchgear, and control
systems.
2. Construction Phase:
 Right-of-Way Clearing:
 Clear vegetation and remove obstacles along the selected route.
 Foundation Excavation and Construction:
 Excavate foundation pits for towers based on soil conditions and tower design.
 Pour concrete foundations to ensure tower stability.
 Tower Erection:
 Assemble tower components on-site and lift them into position using cranes.
 Properly align and secure towers according to design specifications.
 Insulator Installation:
Install suspension insulators on towers with appropriate spacing and stringing
configuration.
 Conductor Stringing:
 Pull conductors along the transmission line route using specialized pulling
equipment.
 Secure conductors to insulators and maintain proper sag and tension.
 Grounding System Installation:
 Install grounding electrodes and conductors to protect against lightning strikes
and fault currents.
3. Substation Construction:
 Site Preparation:
o Clear the substation site and level the ground.
 Equipment Installation:
o Install power transformers, circuit breakers, lightning arrestors, and control
systems.
oConnect substation equipment to the transmission line conductors.
Voltage Level Specific Considerations:
 110kV:
Relatively smaller towers, smaller conductor sizes, and less stringent clearance
requirements.
 220kV:
Larger towers, larger conductor sizes, increased clearance requirements due to higher
voltage.
 400kV:
Very large towers with wider conductor spacing, special design considerations for high
voltage insulation, and strict environmental regulations.
Important Considerations:
 Environmental Impact:

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 Minimize environmental disruption by selecting optimal routes, managing vegetation
clearing, and considering wildlife impact.
 Safety Measures:
Adhere to strict safety protocols during construction, including proper equipment use,
worker training, and live line safety procedures.
 Quality Control:
Implement rigorous quality checks throughout the construction process to ensure
compliance with design specifications
 Terrain: The terrain conditions through which the line runs
 Clearances: The required clearances between the conductors and the tower's axis
 Tower height: The height of the tower, which is determined by the minimum ground
clearance, conductor sag, and more
 Conductor size and material: The size and material of the conductors
 Wind loads: The wind loads that the line will experience
 Temperature variations: The temperature variations that the line will experience

Steps
1. Land work: Prepare the land for the transmission line
2. Mark foundations: Mark the locations where the foundations will be built
3. Prepare foundations: Open up the ground and prepare the foundation base
4. Concrete foundations: Pour concrete into the foundation bases
5. Mount the base pillar: Mount the base pillar part of the tower
6. Install conductors: Install the conductors on the towers
7. Install insulators: Install the insulators between the conductors and the towers

Method of construction of electric supply distribution systems 220 V, 400V, 11 kV, 33 kV

To construct an electric supply distribution system at 220V, 400V, 11kV, and 33kV, typically
follow a process involving: establishing substations to step down the voltage from the
transmission lines, using feeders to distribute power to smaller areas, employing distribution
transformers to further reduce voltage to low levels, and finally connecting individual
consumers through service drops; the key steps include site selection, installation of
transformers, overhead or underground cabling, and protective devices like circuit breakers
and fuses at each stage.
Key Stages:
 Transmission Line Connection:
 Substation Establishment: At the entry point to a distribution area, build a
substation with a step-down transformer to reduce high transmission voltage
(like 132kV or 220kV) to a medium voltage level (33kV or 11kV).
 Switchgear Installation: Equip the substation with circuit breakers, isolators,
and other switchgear to control the power flow and isolate faults.
 Primary Distribution (Medium Voltage):

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  Feeder Lines: From the substation, run overhead or underground cables
(depending on the area) called "feeders" to distribute power to different
localities.
 Conductor Selection: Choose appropriate conductor size based on the
expected load and desired voltage drop.
 Tower Installation: For overhead lines, erect transmission towers to support
the conductors.
 Secondary Distribution (Low Voltage):
 Distribution Transformers: At strategic points within the feeder area, install
distribution transformers to further step down the voltage to 400V or 220V.
 Distributors: From the transformers, run smaller cables called "distributors" to
reach individual consumers or smaller groups of consumers.
 Service Drops: Connect individual buildings to the distributors using service
drops, which are typically single-phase lines for household usage.
Important Considerations:
 Load Analysis:
Conduct a thorough load analysis to determine the required capacity of transformers
and conductors at each stage.
 Voltage Regulation:
Design the system to maintain voltage within acceptable limits by considering conductor
size and placement.
 Protection Devices:
Install appropriate protective devices like fuses, circuit breakers, and lightning arrestors
at each level to safeguard the system.
System Types:
 Radial System:
The simplest design where power flows from the substation through feeders to
consumers without any loops, making it cost-effective but less reliable in case of faults.
 Loop System:
A network of interconnected feeders, providing redundancy and allowing for power
supply even if a section of the line fails.
 Network System:
A combination of loops and radial sections, offering flexibility and improved reliability .

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Main elements of a transmission line
Due to the economic considerations, three-phase three-wire overhead system is
widely used for electric power transmission. Following are the main elements of a
typical power system.

 Conductors: three for a single circuit line and six for a double circuit line.
Conductors must be of proper size (i.e. cross-sectional area). This
depends upon its current capacity. Usually, ACSR (Aluminium-core Steel-
reinforced) conductors are used.
 Transformers: Step-up transformers are used for stepping up the voltage
level and step-down transformers are used for stepping it down.
Transformers permit power to be transmitted at higher efficiency.
 Line insulators: to mechanically support the line conductors while
electrically isolating them from the support towers.
 Support towers: to support the line conductors suspending in the air
overhead.
 Protective devices: to protect the transmission system and to ensure
reliable operation. These include ground wires, lightening arrestors, circuit
breakers, relays etc.
 Voltage regulators: to keep the voltage within permissible limits at the
receiving end.

Module 2

TRANSMISSION LINE PARAMETERS AND PERFORMANCE: Line Parameters: Concepts of R,

L and C of line parameters and types of lines. Performance of short line: Efficiency, regulation and its
derivation, effect of power factor, vector diagram for different power factor. Performance of
medium line:

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Parameters of transmission line

The performance of transmission line depends on the parameters of the line. The transmission line has mainly
four parameters, resistance, inductance, capacitance and shunt conductance. These parameters are uniformly
distributed along the line. Hence, it is also called the distributed parameter of the transmission line.

The inductance and resistance form series impedance whereas the capacitance and conductance form the shunt
admittance. Some critical parameters of transmission line are explained below in detail

Line inductance – The current flow in the transmission line induces the magnetic flux.When the current in the
transmission line changes, the magnetic flux also varies due to which emf induces in the circuit. The magnitude
of inducing emf depends on the rate of change of flux. Emf produces in the transmission line resist the flow of
current in the conductor, and this parameter is known as the inductance of the line.

Line capacitance – In the transmission lines, air acts as a dielectric medium. This dielectric medium constitutes
the capacitor between the conductors, which store the electrical energy, or increase the capacitance of the line.
The capacitance of the conductor is defined as the present of charge per unit of potential difference.

Capacitance is negligible in short transmission lines whereas in long transmission; it is the most important
parameter. It affects the efficiency, voltage regulation, power factor and stability of the system.

Shunt conductance – Air act as a dielectric medium between the conductors. When the alternating voltage
applies in a conductor, some current flow in the dielectric medium because of dielectric imperfections. Such
current is called leakage current. Leakage current depends on the atmospheric condition and pollution like
moisture and surface deposits.

Shunt conductance is defined as the flow of leakage current between the conductors. It is distributed uniformly
along the whole length of the line. The symbol Y represented it, and it is measured in Siemens.

Performance of transmission lines

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The term performance includes the calculation of sending end voltage, sending end current, sending
end power factor, power loss in the lines, efficiency of transmission, regulation and limits of power
flows during steady state and transient conditions. Performance calculations are helpful in system
planning. Some critical parameters are explained below

Voltage regulation – Voltage regulation is defined as the change in the magnitude of the voltage
between the sending and receiving ends of the transmission line.

The efficiency of transmission lines – Efficiency of the transmission lines is defined as the ratio of
the input power to the output power.

Important points

 Admittance measures the capability of an electrical circuit or we can say it measures the
efficiency of a transmission line, to allows AC to flow through them without any obstruction. It
SI unit is Siemens and denoted by the symbol Y.
 Impedance is the inverse of the admittance. Its measure the difficulty occurs in the transmission
line when the AC flow. It is measured in ohms and represented by the symbol z.
 Inductance of Transmission Line

In the medium and long transmission lines inductance (reactance) is more effective than
resistance. The current flow in the transmission line interacts with the other parameter, i.e the
Inductance. We know that when current flow within a conductor, magnetic flux is set up.
With the variation of current in the conductor, the number of lines of flux also changes, and
an emf is induced in it (Faraday’s Law). This induced emf is represented by the parameter
known as inductance.

The flux linking with the conductor consist of two parts, namely, the internal flux and the external flux. The
internal flux is induced due to the current flow in the conductor. The external flux produced around the
conductor is due to its own current and the current of the other conductors place around it. The total inductance
of the conductor is determined by the calculation of the internal and external flux.

Inductance of a two-wire line

Considered a single phase line consisting of two conductors (phase and neutral) a and b of equal radius r. They
are situated at a distance D meters. The cross sections of conductors are shown in the diagram below.

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Let the current flow in the conductors are opposite in direction so that one becomes return path for the other.

The flux linkages of conductor ‘a’ is given by the formula

Here,

Ia = +I
Ib = -I
Daa = r’
Dab = D

Substituting these values in above equation

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Similarly, the flux linkage with the conductor ‘b’ will be

The inductance of the conductor ‘a’

Similarly, the inductance of conductor ‘b’

Inductance per conductor

Inductance of both the conductors is given by the formula

The inductance of an
individual conductor is one-half of the total inductance of a two-wire line.

Inductance of symmetrical three-phase line

In symmetrical three-phase line, all the conductors are placed at the corners of the equilateral triangle. Such an
arrangement of conductors is also referred to as equilateral spacing. It is shown in the diagram below

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Let the spacing between the conductors be D and the radius of each conductor, r. The flux linkages of
conductor a is given by the equation:

In this case

For a three-wire system, the algebraic sum of the currents in the conductors is zero.

So the flux equation becomes

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By using the formula

The inductor of conductor, ‘a’ is

The inductance of conductors b and c will also be the same as that of a. The inductance of the three-phase line is
equal to the two-wire line.

Inductance of unsymmetrical three-phase line

A three-phase line is said to be unsymmetrical when its conductors are situated at different distances. Such
arrangement of conductors is most common in practice because of their cheapness and convenience
in design and construction.

Consider a three-phase unsymmetrical line, having different spacing between their conductors where the radius
of each conductor is r. It is shown in the diagram below

Flux linkage in ‘a’ is expressed by the formula

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Flux linkage in conductor ‘a’ due to ‘b’ is given by the formula

Flux linkage in conductor ‘a’ due to ‘c’ is given by

The average value of flux linkages of ‘a’ is

Since for balanced conditions

By using formula

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The average inductance of phase a is

Similarly,

Thus, it is found that the values of the inductance for the three phases are equalized by transpositions.

Capacitance of Transmission Line

Transmission line conductors constitute a capacitor between them. The
conductors of the transmission line act as a parallel plate of the capacitor and
the air is just like a dielectric medium between them. The capacitance of a line
gives rise to the leading current between the conductors. It depends on the
length of the conductor.

The capacitance of the line is proportional to the length of the transmission line.
Their effect is negligible on the performance of short (having a length less than
80 km) and low voltage transmission line. In the case of high voltage and long
lines, it is considered as one of the most important parameters.

Capacitance of two-wire line

The capacitance of the transmission line along with the conductances forms the
shunt admittance. The conductance in the transmission line is because of the
leakage over the surface of the conductor. Considered a line consisting of two
conductors a and b each of radius r. The distance between the conductors being
D shown in the diagram below:-

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The potential difference between the conductors a and b is

Where, qa – charge on conductor a

qb – charge on conductor b
Vab – potential difference between conductor a and b
ε- absolute permittivity

so that,

Substituting these values in voltage equation we get,

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The capacitance between the conductors is

Cab is referred to as line-to-line-capacitance.

If the two conductors a and b are oppositely charged, and the potential
difference between them is zero, then the potential of each conductor is given
by 1/2 Vab.

The capacitance between each conductor and point of zero potential n is

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Capacitance Cn is called the capacitance to neutral or capacitance to ground.

Capacitance Cab is the combination of two equal capacitance a and b in series.

Thus, capacitance to neutral is twice the capacitance between the conductors,
i.e.,

The absolute permittivity ε is given by

where εo is the permittivity of the free space and εr is the relative permittivity of
the medium.

For air

Capacitance reactance between one conductor and neutral

Capacitance of the symmetrical three-phase line

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Let a balanced system of voltage be applied to a symmetrical three-phase-line
shown below

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The phasor diagram of the three-phase line with equilateral spacing is shown
below:

Take the voltage of conductor a to neutral as a reference phasor

The potential difference between conductor a and b can be written as

Similarly, potential difference between conductors a and c is

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On adding equations (1) and (2), we get

Also,

Combining equation (3) and (4)

From equation (6) and (7)

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The line-to-neutral capacitance

The capacitance of symmetrical three-phase line is same as that of the two-wire

line.

Classification of Transmission Lines

The classification of the transmission lines depends on its voltage and the length
of the conductor. The transmission line is the medium of transferring the power
from the generating station to the load centre. It is mainly classified into two
types. They are the

1. AC Transmission Line
 Short-Transmission Line
 Medium Transmission Line
 Pi Model of a Medium Transmission Line
 T Model of a Medium Transmission Line
 Long Transmission Line
2. DC Transmission Line
The transmission line has resistance R, inductance L, capacitance C and the
shunt or leakage conductance G. These parameters along with the load
and the transmission line determine the performance of the line. The term
performance means the sending end voltage, sending end currents,
sending end power factor, power loss in the line, efficiency of the
transmission line, regulate and limit of power flow during efficiency and
transmission, regulation and limits of power during steady state and
transient condition. The comparison chart of the transmission line is
shown in the figure below.

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Short Transmission Line

A transmission line having its length less than 80 km is considered as a short transmission line. In short
transmission line capacitance is neglected because of small leakage current and other parameters (resistance and
inductance) are lumped in the transmission line.

Single and three phase short transmission line

The single phase line is usually short in length and having low voltage. It has two conductors. Each conductor
has resistance R and inductive reactance X. For convenience, it is considered that the parameters of the

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conductors are lumped into one conductor, and the return conductor is assumed to have no resistance and
inductive reactance.

The single phase line and equivalent circuit model of the short transmission line are shown below in the figure.
The resistance R and the inductive reactance X represent the loop resistance and the loop inductance of the short
transmission line. Thus,

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R = loop resistance of the line = resistance of both outgoing and return conductors
= 2 × resistance of one conductor = 2R1

and X = loop reactance of the lines = reactance of both lead and return conductors
= 2 × inductive reactance to one conductor to neutral = 2X1

The end of the line where the load is connected is called receiving end. The end where the source of supply is
connected is known as the sending end.

Let Vr = voltage at the receiving end

Vs= voltage at the sending end
Ir = current at the receiving end
Is = current at the sending end
cos∅r= power factor of the load
cos∅s = power factor at the sending end

The series impedance of the lines is given as,

In short transmission lines the shunt conductance and shunt capacitance of the line are neglected; hence, the
current remains the same at all point of the line.
Practically, we say that,

The three phase line is made by using three single-phase conductors. Therefore, the calculation remains the
same as explained for the single-phase line, the difference being that per phase basis is adopted. When working
with balanced three phase line, it is assumed that all the given voltages are line-to-line values and all currents
are line currents. Thus, for three phase line calculations,

power per phase = (1/3) × ( total power)

reactive volt-amperes per phase = ( 1/3) × (total reactive volt-amperes)

For a balanced 3-phase, star connected line,

phase voltage = 1/√3 × line voltage

Phasor Diagram

The phasor diagram for a load of lagging power factor is shown below.Let the receiving end voltage V r be taken
as reference phasor, and it is represented by OA in the phasor diagram. For lagging power factor, I lag behind
Vr by an angle ∅r shown in the diagram, where OB = I.

The voltage drop in the resistance of the line = IR. Ir is represented by the phasor AC. It is in phase with the
current and hence drawn parallel to OB. The voltage drop in the reactance of the line is IX and phasor CD
represented it.

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Reactance is lead by 90 degrees and therefore CD is drawn perpendicular to OB. Total impedance voltage drop
IZ is the phasor sum of the resistive and reactive voltage drops, and AD gives it in the diagram.

OD is the sending end voltage Vs, and ∅s is the power factor angle between the sending end voltage and current.
δ is the phase displacement angle between the voltages at the two ends.

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The magnitude of Vs can be found from the right angle triangle OGD.

Power factor of the load measured at the sending end is

If Vr be the reference phasor then,

For lagging power factor cosΦr, I = I <−Φr = IcosΦr −jIsinΦr

For leading power factor cosΦr, I = I<+Φr = IcosΦr + jIsinΦr

For unity power factor, I = I<0° = I + j0°

The line impedance is given by

Sending end voltage is

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For lagging power factor,

ABCD constants of a short line

General equation of the lines for representing voltage and current at the output terminal of the lines is shown
below;

On comparing the output voltage and current of the short line with the above equations, the ABCD constant of
the short line is given below.

The ABCD constants for a short line are given by

Voltage regulation for short lines

It is the change in voltage at the receiving end when the full load at a given power factor is removed and the
voltage at the sending end being constant. It can be written as;

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At full load,

At no load,

Therefore, voltage regulation is given as;

Voltage or
line regulation depends on the power factor. If the line has leading power factor, then the receiving end voltage
is greater and for lagging power factors sending end voltage is greater.

Line Efficiency

It is calculated by the formula given below

Medium Transmission Line

A transmission line having a length of more than 80 kms but less than 250 kms is considered as a
medium transmission line. The parameters (Resistance, Inductance, and Capacitance) are distributed
uniformly along the line. For a medium transmission line, charging current is appreciable and due to
the length of the line the shunt admittance plays a significant role in the calculation of the effective
parameters of the line.

The shunt admittance and series impedance are considered as a lumped parameter of the medium
transmission line. The medium transmission line is shown below in the diagram.

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Many localized capacitance models have been used to make approximate line performance
calculations. The following models are commonly used.

 Nominal T model
 Nominal ∏ (pi) model
 Nominal Pi Model of a Medium Transmission Line

 In the nominal pi model of a medium transmission line, the series impedance of the line is concentrated
at the centre and half of each capacitance is placed at the centre of the line. The nominal Pi model of
the line is shown in the diagram below.

In this circuit,

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By Ohm’s law

By KCL at node a,

Voltage at the sending end

By ohm’s law

Sending-end current is found by applying KCL at node c

or

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Equations can be written in matrix form as

Also,

Hence, the ABCD constants for nominal pi-circuit model of a medium line are

Phasor diagram of nominal pi model

The phasor diagram of a nominal pi-circuit is shown in the figure below.

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It is also drawn for a lagging power factor of the load. In the phasor diagram the
quantities shown are as follows;

OA = Vr – receiving end voltage. It is taken as reference phasor.

OB = Ir – load current lagging Vr by an angle ∅r.
BE = Iab – current in receiving-end capacitance. It leads Vr by 90°.
The line current I is the phasor sum of I r and Iab. It is shown by OE in the
diagram.
AC = IR – voltage drop in the resistance of the line. It is parallel to I.
CD = IX -inductive voltage drop in the line. It is perpendicular to I.
AD = IZ – voltage drop in the line impedance.
OD = Vs – sending–end voltage to neutral. It is phasor sum of V r and IZ.
The current taken by the capacitance at the sending end is I cd. It leads the
sending–end voltage Vs by 90
OF = Is – the sending–end current. It is the phasor sum of I and I cd.
∅s – phase angle between Vs and Is at the sending end, and cos∅s will give the
sending-end power factor.

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Nominal T model of a transmission line
In a nominal T model of a medium transmission line, the series impedance is
divided into two equal parts, while the shunt admittance is concentrated at the
centre of the line. The nominal T model of a medium transmission line is shown
in the figure.

Here,

Series impedance of the line Z = R + jX

Shunt admittance of the line Y = jwc
Receiving end voltage = Vr
Receiving end current = Ir
Current in the capacitor = Iab
Sending end voltage = Vs
Sending end current = Is

Sending end voltage and current can be obtained by application of KVL and
KCL. to the circuit shown below

Current in the capacitor can be given as,

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 By Kirchoff’s current law at node a,

or

By Kirchoff’s voltage law

Equation of Sending end voltage Vs and current Is can be written in the matrix

form as

Also,

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Hence, the ABCD constant of the nominal T-circuit model of a medium line are

The phasor diagram of the nominal T-circuit is shown below. It is drawn for a
lagging power factor.

In the phasor diagram:

OA = Vr – receiving end voltage to neutral. It is taken as a reference phasor.

OB = Ir – load current lagging behind Vr by an angle ∅. cos∅ is the power factor
of the load.
AC = IrR/2 – Voltage drop in the reactance of the right-hand half of the line.It is
perpendicular to OB, i.e., Ir.
OD1 = Vab – voltage at the midpoint of the line across the capacitance C.
BE = Iab – current in the capacitor. It leads the voltage V ab by 90.
OE = Is -sending-end current, the phasor sum of load current and capacitor
current.

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D1C1 = IsR/2 – voltage drop in the resistance on the left-hand side of the lines.It
is perpendicular to Is.
C1D = Is X/2 – voltage drop in the reactance in the left half of the line. It is
perpendicular to Is
OD = Vs – sending end voltage. It is the phasor sum of the of V ab and the
impedance voltage drop in the left-hand half of the line.
∅s – phase angle at the sending end. cos∅s is the power factor at the sending end
of the line.

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Prepared by Prof. Taniya Ghosh
Long Transmission Line

A transmission line having a length more than 240 km is consider as a long transmission line. In a long
transmission line, parameters are uniformly distributed along the whole length of the line. For a long
transmission line, it is considered that the line may be divided into various sections, and each section consists of
an inductance, capacitance, resistance and conductance as shown below.

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Let’s consider a bit smaller part of a long transmission line having length ‘ds’ situated at a distance ‘s’ from the
receiving end. Series impedance of the line is represented by ‘zds’ and ‘yds’ is the shunt impedance of the line.
Due to charging current and corona loss the current is not uniform along the line. Voltage is also different in
different parts of the line because of inductive reactance.

Where, r – resistance per unit length, per phase

l – inductance per unit length, per phase
c – capacitance per unit length, per phase
x – inductive reactance per unit length, per phase
z – series impedance per unit length, per phase
g – shunt leakage conductance, per phase to neutral per unit length
b – shunt leakage susceptance, per phase to neutral per unit length
y – shunt admittance per unit length, per phase to neutral.

For constant supply let,

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V – voltage at a distance ‘s’ from the load end
V + dV – voltage at a distance (s+ds) from the load end
I – current at a distance ‘s’ from the load end
I + dI – current at a distance (s+ds) from the load end.

The difference in the voltage between the ends of the assumed sections of length ds is dV. This difference is
caused by the series impedance of the line.

Similarly, the difference between the two ends of the section resulting from the shunt admittance of the line is
given by the equation

for knowing the value of V, differentiate equation (1) with respect to ‘s’,

and for current differentiate equation

(2)

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 equation (3) and (4) are similar in
form and therefore their general equations are also similar.

The equation (5) is

the linear differential equation with constant coefficients.The general solution of this equations is

Where, C1 and C2 are the arbitrary constants, and it is found from the
known value of the V and I at some point of the line. For determining the value of I differentiate the above

equation with respect to ‘s’

on combining the above equation with equation (1) we get,

substituting
the value of ϒ = √zy in equation (7) gives

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The value of V and I at the receiving end where s = 0, is given by the equations

The values of C1 and C2 are found from the simultaneous equations shown below

and

The value of C1 and C2 are substituted in general equations of voltage and current to obtain the steady-state
values of V and I at any intermediate point distant ‘s’ from the receiving end.

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 and

For governing the behaviour of transmission line in steady-state equation (13) and (14) are used. These
equations can also be written in hyperbolic form by using hyperbolic constant shown below

Substitutes the hyperbolic constant in equations (13) and (14) gives

these equations can also be written as sending end voltage and current equations by replacing s = S

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The ABCD parameters are defined below

These equations help in evaluating the performance of the long line.

DC Transmission Line

The DC transmission is mainly used for the bulk power transmission. For long distance transmission, the DC is
less expensive and have low electrical losses. The cost of the DC transmission systems is higher for short
distance transmission line because it requires more convertible equipment as compared to an AC system.

HVDC TRANSMISSION LINE

The converter station converts the AC to DC at the sending end and DC to AC at the load end of the line. One of
the major advantage of the DC system is that it allows the power transmission between two unsynchronised AC
system.

Transposition of Conductors

Definition: The transposition is a physical rotation of the conductors so that the conductor is moved to take up
the next physical position in the regular sequence. The transposition of the conductor equalises the
mutual inductance and capacitance between the lines. The irregular spacing between the conductor gives a
complex value of inductances which makes the study of the power system complex. The transposition is mainly
done in the switching station and the substations. The transposition cycle is shown in the figure below.

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Needs of Transposition

The inductance of unsymmetrical line causes voltage drops even if the voltage is in a balanced condition.
Because of the inducing voltages, the magnetic field exists in the conductor which causes the interference in the
line. This can be reduced by continually exchanging the position of the conductor, which can be done by
transposition the conductors.

Transposition Method

The transposition of the lines can be done by placing the one-third segment of all three phases of the conductor
in the same line. The transposition balance the capacitance of the line due to which the voltage is also balanced.

Disadvantage of Transposition

Frequently changing the position of conductors weakens the supportive structure which increases the cost of the
system.

Why Transposition Arrangement is Use

The transposition under power lines reduce the electrostatic unbalance among the three phases. it also used to
stabilize the voltage unbalance.

The transposition arrangement of high voltage lines also helps to reduce the system power loss.

In addition to this, we have developed Transposition System for Single circuit tower using same tension tower
with reduced deviation angle.

Transposition arrangement of power line also helps to reduce the effect of inductive coupling.

So would be benefited by reducing one more tower design, approval, and results in a reduction of overall project
time, etc.

It is proved more economical Solution, in comparison of the conventional transposition system.

The term “skin effect” has a wide role in AC power system, and people have many doubts regarding
this topic, but this post will make this topic very easy for you, we will learn that What is Skin Effect?,
Why skin effect occurs?, Why skin effect does not occur in DC?, and How to reduce Skin Effect?

What is Skin Effect?

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When AC current pass through a conductor then it is observed that, the majority of the current will
flow from the outer part/skin/surface of the conductor.

Definition: The tendency of alternating current to flow near the surface or skin of a conductor is
known as Skin Effect.

You can see the phenomenon of skin effect in below figure, where charge concentration is present
more near the surface or skin of the conductor, or we can say that, the whole cross-section of the
conductor is not being used to flow the current through it.

We have seen that What is skin effect? But now question arises that Why or How does skin effect
occur? Let’s discuss it.

Why does skin effect occur?

Let us take the cross-sectional view of a conductor and we assume this conductor to be made up of
several concentric cylinders as shown in the fig. below, and each of its layer will carry some amount
of current.

We know that, when an alternating current pass through a conductor/wire, then it produces a
magnetic field around it and direction of this magnetic field is given by “Right-Hand Thumb Rule” as
shown in fig. below.

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As our conductor has several current carrying layers and each of its layer will create its own magnetic
field around that layer.

Now, these magnetic field lines will create magnetic flux (ɸ). Also, we know that when magnetic flux
is multiplied by the no. of turns/coils/layers (N ɸ), then it becomes Magnetic Flux Linkage (ψ).

It is observed that the magnetic flux at the core or central part of the conductor is stronger than the
outer part of the conductor.

To understand the phenomenon of skin effect, we are going to study about two extreme layers of the
conductor (central or inner layer and outer/surface layer).

As we have seen that, magnetic flux is stronger at the central part of the conductor then it is quite
obvious that magnetic flux linkage (ψ) will also be more for the inner layer (layer 1) than outer layer
(layer 2).

Now, we know that, Inductance is directly proportional to the magnetic flux linkage (ψ) by the
formula given below.

By the above relation between magnetic flux linkage and Inductance (L), It is clear that, Inductance at
the first layer is more than the layer 2 (outer layer).

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Reactance (XL) is directly related to the inductance (L). As the formula of reactance is X L = 2π f L.

It means that, for layer 1 reactance will be high as compared to the layer 2.

Now Impedance is given by,

Z = R + j XL

here, Resistance is a uniformly distributed parameter and it does not depend on the value of flux, so
Impedance is directly depending on the value of X L.Where Reactance is low, the Impedance will also
be low.

We know that, I = V/Z

With the help of above formula, we can say that if the value of Z increases, then the value of Current
will decrease.

For layer1, the Impedance is very High, so the current in the layer1 will be very low. And, for Layer 2,
the Impedance is very low, so current in the outer layer will be high.

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With the help of above table, we can see that Layer1 (Inner layer) has very less current and Layer2
(outer layer) has more current. And we called this phenomenon as Skin Effect.

Cause of skin effect: The non-uniform magnetic flux (linkage) is the root cause of the skin effect.

Why skin effect does not occur in DC?

In case of DC, the frequency is zero. (f=0). Hence, XL will also be zero, because the formula of XL is,

XL = 2π f L

XL = 2π (0) L = 0

Therefore, Impedance (Z) will only depend on the resistance,

Z = R + j XL

Z=R+0

Z=R

And we know that, resistance is a uniformly distributed parameter, therefore the current flowing
through the wire will also uniformly distributed and that’s why skin effect does not occur in case of
DC.

Disadvantages of skin effect

Skin effect is a undesired phenomenon which leads to many disadvantages in the power system as:

1. Effective resistance increases (Reff ↑)

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Due to skin effect, majority current flows near the surface of the conductor. Due to this phenomenon
the whole cross-section of the conductor is not being used and this reduces the effective cross-
sectional area (the area of the conductor through which the current is actually flowing).

As we know that resistance is inversely proportional to the area, If the effective cross-sectional area is
reduced then the effective resistance will increase.

2. Copper losses Increases:

Copper losses or I2R losses will increase because the effective resistance has increased due to ski
effect. Increment in copper losses also leads to Heat loss.

3. Voltage drop increases:

When resistance increases, it also increases the Voltage drop.

4. Voltage regulation decreases

5. Efficiency decreases

What are the factors which affect the skin effect?

1. Supply frequency:

Supply frequency is directly proportional to the skin effect. Because, Reactance is directly
proportional the supply frequency (XL = 2π f L)

2. Diameter of the conductor:

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Skin effect is directly proportional to the Diameter of the conductor. Skin effect is very less for thin
conductors and very high for thick conductors.

3. Nature of material:

Relative permeability (µr) is a property of material which supports the formation of magnetic field
and skin effect is directly proportional to the Relative permeability (µr).

If µr of a material has high value, then it will support the formation of magnetic flux which will
increase the Inductance, Reactance and Impedance, and decrease the current in the inner section of
conductor. In other words, we can say that Skin effect will be more if Relative permeability of a
material is high.

4. Type of Conductor:

Skin effect is more in case of Solid conductors and Skin Effect is less in case of stranded conductors
and Hollow Conductors.

Read here: How Stranded conductor or ACSR conductor reduces the Skin effect.

5. Temperature:

Skin effect increases by increasing the temperature. It means that Skin effect is directly proportional to
the Temperature.

Methods to reduce skin effect

We can reduce the skin effect to some extent but we can not Finish the skin effect, There are few
methods to reduce the Skin effect a follows:

1. By using Stranded conductors:

Stranded conductor consists of several thin wires of small cross-sectional area called as Strands. These
conductors are more flexible than the solid conductors and they help to reduce the skin effect.

This is the most widely used method to reduce the skin effect. There are many types of Stranded
conductors but generally we use ACSR (Aluminium Conductor Steel- Reinforced) conductor.

This conductor is made up of two layers, the inner layer has Steel strands which provide strength to
the cable, and outer layer is made up of Aluminium strands which are used to take current as shown
in the figure below.

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2. Reducing the Diameter:

By reducing the thickness of conductor we can reduce the Skin effect.

3. Using hollow conductors:

As we know that, Due to skin effect the majority of the current flows through the outer surface of the
conductor, so if we use the Hollow conductor then We can eliminate the waste material of the
conductor. This method is only used in some applications like Bus-Bars in substation. As, Hollow
conductors are very costly to design and they have very low mechanical strength, so they can not be
used in the long-distance Transmission.

Few more methods like, Reducing the frequency, reducing the Temperature can also reduce skin
effect.

The Proximity Effect in Transmission Lines

AuthorCadence System Analysis

Key Takeaways

 An increase in apparent resistance in a conductor causes a voltage drop and power loss. This
phenomenon is called the proximity effect.
 A conductor's material, diameter, and structure all influence the intensity of the proximity
effect.
 It is possible to reduce the proximity effect by reducing the size of the conductor and the
frequency and by increasing the voltage and space between conductors.

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 

The proximity effect is present in transmission lines when conductors are too close together

Delta-connected ac transmission lines transmit three-phase ac power between substations. When

conductors are too close to each other in a delta arrangement, the proximity effect is present in

transmission lines. The proximity effect could be avoided by keeping conductors spaced equally.
However, extending the distance between transmission lines inflates the expense of the support
structures, directly affecting the efficiency of the ac power transmission. In this article, we will discuss
how to reduce the influence of the proximity effect in transmission lines.

The Proximity Effect in Transmission Lines

Conductors carrying alternating current will produce alternating flux in adjacent conductors. This
alternating flux will cause a circulating current to start flowing in the conductor, creating a non-uniform
current distribution in the transmission line, increasing the conductor's apparent resistance. The
increased resistance along the transmission line causes a voltage drop and power loss. This phenomenon
is called the proximity effect.

How Does the Proximity Effect Impact Transmission Lines?

The concentration of current through adjacent conductors varies with the alternating magnetic field and
its associated eddy currents. When conductors carry current in the same direction, the currents flowing
through them get concentrated at the conductors' farthest side. In contrast, when currents flowing
through adjacent conductors flow in opposite directions, the currents get concentrated in the nearest side
of both conductors.

As the alternating current frequency increases, the proximity effect becomes more intense. Conductors
carrying 50Hz current endure less of the proximity effect than conductors carrying 60Hz current. The

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effective resistance and power loss is higher in 60Hz transmission lines than in 50Hz transmission lines.
Most countries worldwide use 50Hz ac frequency, but the United States is not one of them. The 60Hz
frequency in the transmission line causes more of the proximity effect than the 50Hz supply.

The proximity effect is due to varying magnetic fields, making it an impossible phenomenon in dc
transmission. As dc frequency is zero, it fails to produce an alternating magnetic field in adjacent
conductors. The current concentration remains uniform in dc transmission lines, apart from the influence
of the skin effect.

Factors Influencing the Proximity Effect

Both transmission lines and nearby conductors carrying alternating currents experience the proximity
effect. In ac transformers and inductors, the windings are close enough that the proximity effect is more
predominant than the skin effect. If the conductors are stranded, both the internal proximity effect and
external proximity effect exist. Several factors influence the proximity effect in transmission lines,
including:

1. The conductor’s material - High ferromagnetic materials experience more proximity

effects than non-ferromagnetic materials.
2. The conductor’s diameter - As the conductor’s diameter increases, the proximity effect
also increases. The conductor’s diameter is dependent on current, and when the system
current is high, the proximity effect becomes stronger.
3. Frequency - As the frequency increases, the proximity effect becomes more intense.
4. The conductor’s structure - The proximity effect is higher in solid conductors than in
stranded conductors. The decreased surface area of stranded conductors causes the
proximity effect to be less than in solid conductors, which have more surface area.
However, the internal proximity effect and external proximity effect exist in stranded
conductors such as ACSR.

How to Reduce the Proximity Effect

Knowing the factors that create the proximity effect in transmission lines, it is possible to implement
some changes. Several fixes can reduce the influence of the proximity effect, which include:

1. Reducing the size of the conductor - The proximity effect is directly proportional to the
surface area of the conductor. Therefore, as the surface area increases, the proximity
effect becomes stronger. Replacing solid conductors with stranded conductors helps
reduce the conductor's surface area, decreasing the proximity effect.
2. Increasing the space between conductors - Dummy conductors can help increase the
space between conductors. However, this will come at an added cost in support
structures.
3. Increasing voltage and reducing frequency - Transferring power constantly through
transmission lines increases voltage and decreases current—the reduced size of

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 conductors decreases the proximity effect. Although not as practical, reducing the
transmission voltage and current frequency is another means of reducing the proximity
effect.
The proximity effect in transmission lines is a limitation in electrical ac power transmission. The
proximity effect is present not only in high voltage systems, but also in medium and low voltage systems.
Cadence’s software can help reduce the proximity effect in microstrips or striplines in circuit boards.

MODULE 3

EXTRA HIGH VOLTAGE TRANSMISSION: Extra High Voltage AC (EHVAC) transmission line:
Necessity, high voltage substation components such as transformers and other switchgears,
advantages,limitations and applications and lines in India. Ferranti and Corona effect.
High Voltage DC (HVDC)
Transmission Line: Necessity, components, advantages, limitations and applications. Layout of
monopolar, bi-Polar and homo-polar transmission lines. Lines in India. Features of EHVAC and
HVDC
transmission line. Flexible AC Transmission line: Features, d types of FACTS controller. New trends
in
wireless transmission of electrical power.

Extra high voltage (EHV) alternating current (AC) transmission lines are necessary because they can transport
electricity over long distances with minimal loss. EHV transmission lines are more efficient, economical, and
reliable than lower voltage lines.
Benefits of EHV transmission lines

 Increased efficiency: EHV transmission lines reduce energy loss due to conductor resistance.
 Increased transmission capacity: EHV transmission lines can increase the amount of power that can
be transmitted over a line.
 Improved voltage regulation: EHV transmission lines can help manage voltage drop and ensure grid
stability.
 Better load balancing: EHV transmission lines can help balance the load on the power grid.
 Economical: EHV transmission lines can be economical for connecting power systems on a large
scale.
EHV transmission line voltage levels

 EHV transmission lines are typically 345 kV and above.

 Internationally, EHV transmission lines are usually classified as having a nominal voltage from 330 kV to
below 1000 kV.
 Transmission of bulk power from generating stations to the load centers is technically and economically
feasible only at voltages in the EHV/UHV range. An EHVAC Transmission line stands for Extra High
Voltage Alternating Current Transmission line. EHV lines are able to move large power across a single
line so the overall conductor requirement is decreased.
 EHVAC transmission across the world at voltages 220kv, 400kv, and 765kv.

 1. Necessity for EHV AC Transmission

 Long-Distance Power Transfer:

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 For regions with power plants located far from population centers, EHVAC lines are
essential to efficiently transmit electricity over large distances.
 Grid Stability:
By enabling efficient power transfer, EHVAC lines contribute to grid stability and load
balancing, ensuring reliable electricity supply even during peak demand periods .

 1. With the increase in transmission voltage, for same amount of power to be transmitted current in the
line decreases which reduces I2R losses (or copper losses). This will lead to increase in transmission
efficiency.
 2. With decrease in transmission current, size of conductor required reduces which decreases the
volume of conductor.
 3. The transmission capacity is proportional to square of operating voltages. Thus the transmission
capacity of line increases with increase in voltage. The costs associated with tower, insulation, and
different equipments are proportional to voltages rather than square of voltages. Thus the overall
capital cost of transmission decreases as voltage increases. Hence large power can be economically
transmitted with EHV or UHV.
 4. With increase in level of transmission voltage, the installation cost of the transmission line per km
decreases.
 5. It is economical with EHV transmission to interconnect the power systems on a large scale.
 6. The number of circuits and the land requirement for transmission decreases with the use of higher
transmission voltages.
 7. Large amounts of power over long distances is technically and economically feasible only at voltages
in EHV and UHV range. Thus Economics can be achieved in power generation.

 Advantages of EHV AC Transmission System

 Electrical energy is generated at a voltage about 11 kV using alternators. This voltage is then stepped
up to 132, 220 or 400 kV for transmission purpose. For transmission of electric power high voltage is
preferred because of following advantages,
 1) Reduction in the current
 Power transmitted is given by
 P = √ 3 VLIL cos ϕ
 where VL = Line voltage, IL = Load line current
 cos ϕ = Load power factor
 Hence load current is given by, IL = P = √ 3 VL cos ϕ
 From the above expression it can be seen that for the constant power and power factor, the load current
is inversely proportional to the transmission voltage. With increase in transmission voltage, load current
gets reduced. As current gets reduced, size of conductor required also reduces for transmitting same
amount of power, which reduces the cost.

Prepared by Prof. Taniya Ghosh

  2) Reduction in the losses
 Power loss in a line is given by,


 From the above expression it can be seen that power loss in a line is inversely proportional to square of
transmission voltage i.e. greater the transmission voltage lesser is the loss in the line.
 3) Reduction in volume of conductor material required
 We have seen that,


 Volume of conductor meterial required = 3 × Area of conductor × Length of line


 It can be seen that with increase in the transmission voltage, volume of conductor material reduces.
 4) Decrease in voltage drop and improvement of voltage regulation.
 The voltage drop in the transmission line is given by,
 Voltage drop = 3 I R
 With reduction in current due to increase in voltage, voltage drop in the line reduces.
 Voltage Regulation = Voltage drop / Sending voltage × 100
 As voltage drop decreases, regulation of the line is improved.
 5) Increase in transmission efficiency
 Transmission efficiency is given by,
 Transmission efficiency = Output power / Input power × 100
 = Input power - Power loss / Input loss × 100 = (1 - Power loss / Input power ) × 100
 We have seen that with increase in transmission line voltage power loss gets reduced. Hence the
transmission efficiency increases as losses in the line are reduced.

Prepared by Prof. Taniya Ghosh

  6) Increased power handling capacity
 Power transmitted over a transmission line is given by,
 P = VS . VR / X sin δ
 Thus if we assume that VS = VR then power transmitted is proportional to square of voltage which
increases power handling capacity of the line.
 7) The number of circuits and the land requirement reduces as transmission voltage increases.
 8) The total line cost per MW per km decreases considerably with the increase in line voltage.
 9) The operation with EHV AC voltage is simple and can be adopted easily and naturally to the
synchronously operating a.c. systems.
 10) The equipments used in EHV AC system are simple and reliable without need of high technology.
 11) The lines can be easily tapped and extended with simple control of power flow in the network.


 4. Disadvantages or Problems involved in EHV AC Transmission System

 The major problems that can be occurred with EHV transmission system are as follows
 1) Corona loss and radio interference
 The corona loss is greatly influened by choice of transmission voltage. If weather conditions are not
proper then this loss further increases. There is also interference in radio and TV which causes
disturbance.
 2) Line supports
 In order to protect the transmission line during storms and cyclones and to make it wind resistant, extra
amount of metal is required in the tower which may increase the cost.
 3) Erection difficulties
 There are lot of problems that arise during the erection of EHV lines. It requires high standard of
workmanship. The supporting structures are to be efficiently transported.
 4) Insulation needs
 With increase in transmission voltage, insulation required for line conductors also increases which
increases its cost.
 5) The cost of transformers, switchgear equipments and protective equipments increases with increase
in transmission line voltage.
 6) The EHV lines generates electrostatic effects which are harmful to human beings and animals.


 5. Environmental Considerations for EHV AC Transmission

 The various environmental considerations for EHV AC transmission system are,

Prepared by Prof. Taniya Ghosh

  1. Corona effect and ozone gas discharge at the time of corona. It affects the sun and hence affects the
environment. So corona effect must be reduced.
 2. Radio and television interference is generated due to corona which causes disturbance in wireless
signals and communication lines. In bad weather conditions the corona is more and radio interference
is more. The radio interference plays an important role in designing of EHV AC lines.
 3. For a large voltage, a hissing sound is generated due to corona which can be easily heard and affects
the environment. The humming noise from transformers and other electrical equipments also create
audible noise. The care must be taken to keep such audible noise as low as possible.
 4. Practically EHV AC lines run through forests, farm lands and hilly areas. Thus clearing a path for these
lines is an important aspect without affecting environmental balance. The possibility of fire due to the
branches of dead trees near such lines is another issue. Such trees and branches must be cut and
removed.
 5. EHV AC lines are responsible to produce electromagnetic and electrostatic fields which are harmful
to human and animals. These fields produce adverse effects of human health such as changes in
immune system, changes to the functions of cells and tissues, inducing currents on the surface of the
human body, changes in the heart rate and brain activity of human etc. The efforts must be taken to
reduce such fields so as to restrict their biological effects.
 6. Proper protective equipments must be provided to reduce the effects of lightning, storms and other
adverse atmospheric conditions on the environment.


 6. Standard Rated Voltages of EHVAC Lines

 The standard rated voltages for AC transmission are given in the Table 7.24.1.


 The choice for the transmission line voltage is made by referring this table. For a new line, the chioce
of voltage is made in such a way that the nearest existing system voltage is preferred.
 In EHVAC lines additional parallel three phase line is always provided to maintain continuous flow of
power and stability of transmission line.

In a high voltage substation, key components include power transformers, circuit breakers, disconnectors,
lightning arrestors, current transformers (CTs), potential transformers (PTs), busbars, earthing switches, and
control and protection relays; all designed to safely manage and distribute high voltage electricity within the
EHVAC (Extra High Voltage Alternating Current) system.

Prepared by Prof. Taniya Ghosh

Explanation of key components:

 Power Transformers:
The primary component, responsible for stepping up or down the voltage levels in the transmission
network.
 Circuit Breakers:
Act as switching devices to interrupt the circuit during fault conditions, providing protection against
overcurrent situations.

 Disconnectors:
Used to isolate sections of the circuit for maintenance or repairs, but cannot interrupt current under load.
 Lightning Arrestors:
Protect equipment from voltage surges caused by lightning strikes by diverting excess voltage to the
ground.

 Current Transformers (CTs):

Measure the current flowing in a circuit by stepping down the high current to a lower level suitable for
measuring instruments.
 Potential Transformers (PTs):
Measure the voltage in a circuit by stepping down the high voltage to a lower level for measurement.

 Busbars:
Conductors that serve as the main collection point for power distribution within the substation.
 Earthing Switches:
Provide a connection to the ground for safety purposes, particularly during maintenance.
 Control and Protection Relays:
Monitor system conditions and initiate protective actions like tripping circuit breakers in case of faults.

Configuration of EHV A.C. Transmission

The typical configuration of a very long EHV / UHV three phase AC transmission
system is shown in the Fig.

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EHV AC transmission line requires minimum two parallel three phase
transmission circuits to ensure reliability and stability during a fault on any one
phase of the three phase lines.

Similarly EHV line also requires one or more intermediate substations for
installing series capacitors, shunt reactors, switching and protection equipment.

Extra High Voltage AC (EHVAC) transmission lines, the "Ferranti effect"

refers to a phenomenon where the voltage at the receiving end of a line
becomes higher than the sending end voltage when operating under light
load or no load conditions, caused by the interaction between the line's
capacitance and inductance; while the "Corona effect" is the ionization of
the air surrounding a conductor due to high electric fields, causing a
luminous glow and potential power loss, often mitigated by using corona
rings on the conductors.
Key points about each effect:
 Ferranti Effect:
 Cause: When a transmission line is lightly loaded, the capacitive
current flowing through the line becomes significant, leading to a
voltage rise at the receiving end.
 Impact: Can potentially damage equipment connected to the
receiving end if not managed properly.
 Mitigation: Installing shunt reactors at the receiving end to
counteract the capacitive current.
 Corona Effect:
 Cause: When the electric field around a conductor becomes
strong enough, it ionizes the surrounding air, creating a visible
glow and generating a hissing sound.
 Impact: Power loss due to corona discharge, potential for radio
interference.
 Mitigation: Using larger diameter conductors, installing corona
rings on the conductors to distribute the electric field more
evenly.

Prepared by Prof. Taniya Ghosh

What is the Corona Effect in Transmission Lines?

There is a hissing noise with violet glow phenomenon termed as corona effect which is commonly observed in
high voltage transmission lines. The corona effects leads to high voltage drop and energy loss along with release
of ozone gas. There is a need to be aware of this phenomenon and its effects on the transmission system. Read
more as we cover the factors contributing to corona effect, its disadvantages & also some advantages and
methods to reduce the corona effect.

What is Corona Discharge?

DEFINITION: The ionization of air surrounding the high voltage transmission lines causing the conductors to glow,
producing a hissing noise, is called Corona Discharge or Corona Effect.

Corona Effect in Transmission Lines:

This phenomenon occurs when the electrostatic field across the transmission line conductors produces the
condition of potential gradient. The air gets ionized when the potential gradient at the conductor surface reaches
the value of 30kV/cm at normal pressure and temperature.

In transmission lines, conductors are surrounded by the air. Air acts as a dielectric medium. When the electric field
intensity is less than 30kV/cm, the induced current between the conductor is not sufficient to ionize the air. However,
when the voltage of air surrounding the conductor exceeds the value of 30kV/cm, the charging current starts to
flow through the air, that is air has been ionized. The ionized air act as a virtual conductor, producing a hissing
sound with a luminous violet glow.

Advantages & Disadvantages of Corona Effect

Prepared by Prof. Taniya Ghosh

1. Advantages:

To every disadvantage, there is a corresponding advantage. Corona effect may highly affect the efficiency of
transmission lines, however, it also provides safety to the transmission line.

The main advantages of corona effects are:

 Due to corona across the conductor, the sheath of air surrounding the conductor becomes conductive
which rises the conductor diameter virtually. This virtual increase in the conductor diameter reduces the
maximum potential gradient or maximum electrostatic stress. Thus, the probability of flash-over is
reduced.
 Effects of transients produced by lightning or electrical surges are also reduced due to the corona effect.
As, the charges induced on the line by surge or other causes, will be partially dissipated as a corona loss.
In this way, corona protects the transmission lines by reducing the effect of transients that are produced
by voltage surges.

2. Disadvantages:

The corona effect has the following disadvantages:

 A non-sinusoidal voltage drop occurs in the transmission line due to non-sinusoidal corona current,
which causes interference with neighboring communication circuits due to electromagnetic transients
and electrostatic induction effects.
 Ozone gas is produced due to the formation of corona, which chemically reacts with the conductor and
causes corrosion.
 The energy dissipated in the system due to corona effect is called as Corona loss. The power loss due
to corona is undesirable and uneconomical. The efficiency of transmission line is highly reduced due to
the loss of power or energy.

We can protect our power system from electric transients by using various devices. Read our blog on Surge
Protection Devices which help to mitigate the effect of such transients.

Factors Affecting Corona Discharge:

The phenomenon of electric discharge associated with energized electrical devices, including transmission lines
results in a power loss, reducing the efficiency of the transmission lines. The following factors can change the
magnitude of the Corona Effect:

1. Supply Voltage: As the electrical corona discharge mainly depends upon the electric field intensity
produced by the applied system voltage. Therefore, if the applied voltage is high, the corona discharge
will cause excessive corona loss in the transmission lines. On contrary, the corona is negligible in the low-
voltage transmission lines, due to the inadequate amount of electric field required for the breakdown of
air.
2. Conductor Surface: The corona effect depends upon the shape, material and conditions of the
conductors. The rough and irregular surface i.e., unevenness of the surface, decreases the value of
breakdown voltage. This decrease in breakdown voltage due to concentrated electric field at rough spots,
give rise to more corona effect. The roughness of conductor is usually caused due to the deposition of
dirt, dust and scratching. Raindrops, snow, fog and condensation accumulated on the conductor surface
are also sources of surface irregularities that can increase corona.
3. Air Density Factor: Air density factor also determines the corona loss in transmission lines. The corona
loss in inversely proportional to air density factor. Power loss is high due to corona in Transmission lines
that are passing through a hilly area because in a hilly area the density of air is low.

Prepared by Prof. Taniya Ghosh

 4. Spacing between Conductors: Design engineers calculate the spacing between the two conductors in
the transmission line after careful and extensive research. As the phenomenon of corona discharge is
affected by the conductor spacing. If the distance between two conductors is very large as compared to
the diameter of conductor, the corona effect may not happen. It is because the larger distance between
conductors reduces the electro-static stress at the conductor surface, thus avoiding corona formation.
5. Atmosphere: As corona is formed due to ionization of air surrounding the conductors, therefore, it is
affected by the physical state of atmosphere. In the stormy weather, the number of ions is more than
normal weather. The decrease in the value of breakdown voltage is followed by the increase in the number
of ions. As a result of it, corona occurs at much less voltage as compared to the breakdown voltage value
in fair weather.

By Iantresman - CC BY 3.0, Link

How Corona Effect is Reduced:

It has been observed that the intense corona effects are observed at a working voltage of 33 kV or above. On the
sub-stations or bus-bars rated for 33 kV and higher voltages, highly ionized air may cause flash-over in the
insulators or between the phases, causing considerable damage to the equipment, if careful designing is not made
to reduce the corona effect.The corona effect can be reduced by the following methods:

1. By Increasing Conductor Size: The voltage at which corona occurs can be raised by
increasing conductor size. Hence, the corona effect may be reduced. This is one of the reasons that ACSR
conductors which have a larger cross-sectional area are used in transmission lines.
2. By Increasing Conductor Spacing: The corona effect can be eliminated by increasing the spacing
between conductors, which raises the voltage at which corona occurs. However, increase in conductor
spacing is limited due to the cost of supporting structure as bigger cross arms and supports to accompany
the increase in conductor spacing, increases the cost of transmission system.
3. By Using Corona Ring: The intensity of electric field is high at the point where the conductor curvature
is sharp. Therefore, corona discharge occurs first at the sharp points, edges, and corners. In order to,
mitigate electric field, corona rings are employed at the terminals of very high voltage equipment.

Corona rings are metallic rings of toroidal shaped, which are fixed at the end of bushings and insulator strings.
This metallic ring distributes the charge across a wider area due to its smooth round shape which significantly
reduces the potential gradient at the surface of the conductor below the critical disruptive value and thus preventing
corona discharge.

Prepared by Prof. Taniya Ghosh

 To grab information about conductor sizing, read our previous blog on how to choose the most economic size
and type of cable?

Important Parameters in Corona Analysis:

In the design of an overhead transmission line, the phenomenon of corona plays an important role. Therefore, the
following terms are used in the analysis of corona effects:

1. Critical Disruptive Voltage: The minimum phase-neutral voltage at which corona occurs is known as 'Critical
disruptive voltage'.

Now, consider two conductors having radius of 'r' cm separated from each other by 'd' cm. Potential gradient 'g' at
the conductor surface is given by:

Where, V is phase-neutral potential. The value of g must be made equal to the breakdown strength of air, for the
formation of corona.The breakdown strength of air at 76 cm pressure and temperature of 25ºC is 30 kV/cm (max)
or 212 kV/cm (r.m.s.) and is denoted by go.If Vc is the phase-neutral potential required to produce corona under
these conditions, then,

Prepared by Prof. Taniya Ghosh

 Therefore, Critical disruptive voltage is:

The value of go is directly proportional to air density.Thus, the breakdown strength of air at a barometric pressure
of b cm of mercury and temperature of t 0C becomes:

Under standard conditions, the value of δ = 1.

∴ Critical disruptive voltage, Vc = go δ r loge (d/r)

The corona effect also depends upon the surface condition of the conductor. Thus, the irregularity factor mo is
accounted by multiplying the above expression.

∴ Critical disruptive voltage, Vc = mo go δ r loge (d/r) kV/phase

Where, value of mo is given as:

S.No. Conductor surface Value of irregularity factor

1 Polished conductors 1
2 Dirty conductors 0.92 - 0.98
3 Stranded conductor 0.8 - 0.87

2. Visual Critical Voltage: At disruptive voltage Vc, the glow of corona does not appear along the conductors but
at a higher voltage Vv, termed as Visual critical voltage. The phase-neutral effective value of visual critical voltage
can be determined by the following formula:

Where m v is another irregularity factor. Its value is 1 when conductors' surface is polished, and 0.72 to 0.82 for
rough conductors.

3. Power Loss Due to Corona: The formation of corona is always accompanied by the loss of energy which is
dissipated in the form of light, heat, sound and chemical action. When disruptive voltage is exceeded, the power
loss due to corona is given by:

Prepared by Prof. Taniya Ghosh

Where,

f = supply frequency in Hz

V = phase-neutral voltage (r.m.s.)

Vc = disruptive voltage (r.m.s.) per phase

In all, electrical corona discharge is an important factor in transmission and sub-transmission systems which should
be taken into account to ensure both reduction in energy loss and increment in system safety. Corona effect causes
corrosion at conductor's surface, and pose a threat to the signal integrity of data communication. Several
techniques have been implemented to reduce the corona effect to some extent. Such as some present day methods
include increasing the conductor's diameter, spacing between the transmission line conductors, using hollow
conductors and corona rings.

Generally, it is recommended to use Aluminum corona rings at the conductor end of the string insulators for lines
above 230 kV and on both ends of the insulator for 500 kV. Having a good understanding of the nature of conductor
material is essential for engineers, so that they can develop proper techniques to mitigate adverse effects caused
by electrical corona. In the end, it is emphasized to consider the configuration of the conducting line as well as the
factors affecting corona discharge such as, resistance of the AC line and the current capacity.

Prepared by Prof. Taniya Ghosh