Swami Keshvanand Institute of Technology, Management
& Gramothan, Jaipur
Department of Electrical Engineering
Lecture-1
Basics of AC Transmission
Objectives of Lecture

To study Transmission network basics and understand
the concept of power transmission through various
circuit elements.

To understand Ac transmission networks and its stages.

To compare different transmission networks.

To learn advantages
transmission networks
Lecture-1 (Power Quality and Facts)
and
disadvantages
of
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AC
2
Basics of AC Transmission

Electric power transmission systems are the means
of transmitting power from a generating source to
various load centers (i.e. where the power is being
used).

These generating stations are not necessarily
situated near consumers (i.e. the load center).

Overall, electrical supply systems are the network
through which consumers of electricity receive
power from a generation source.

Power transmission systems transport the power
from the generation source and into a power
distribution system.
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Transmission Elements
The transmission line is a closed system through which the
power is transfer from generating station to the consumers. The
transmission lined are categorized into three categories.

Short Transmission Line – The length of the short
transmission line is up to 80Km.

Medium Transmission Line – The length of the medium
transmission line lies between 80km to 200km.

Long transmission Line – The length of long transmission
line is greater than 150km.

The supports conductor, conductor, insulator, cross arms
and clamp, fuses and isolating switches, phases plates etc.
are the main component of the transmission lines.
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AC power transmission system
• Electric power is commonly (or usually)
generated at 11 kV in generating stations in
India and Europe. While in some cases,
generation voltage might be higher or
lower. Generating machines, to be used in
power stations, are available between 6 kV
to 25 kV from some big manufacturers
(Primary Transmission).
• The voltage is the stepped down at a
receiving station to 33kV or 66kV.
(Secondary transmission lines) emerge
from this receiving station to connect
substations located near load centers (cities
etc.).
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Main elements of
Transmission Systems

Transformers

Line insulators

Support towers

Protective devices

Voltage regulators
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Types of Transmission Networks

Different types of transmission systems
Single phase AC system

single phase, two wires

single phase, two wires with midpoint earthed

single phase, three wires
Two phase AC system

Two-phase, three wires

Two-phase, four wires
Three phase AC system

three-phase, three wires

three-phase, four wires
DC system

DC two wires

DC two wires with midpoint earthed

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DC three wires
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AC Vs DC Transmission
Basis for
Comparison
Definition
Number of
Conductors
Inductance & surges
Voltage drop
Skin Effect
Need of Insulation
Interference
Corona Loss
Dielectric Loss
Synchronizing and
Stability Problem
Cost
Length of conductors
Repairing and
Maintenance
Transformer
AC Transmission Line
DC Transmission Line
The ac transmission line - The dc transmission line is
for transmitting the
used for transmitting the
alternating current.
direct current.
Three
Two
Present
High
Present
More
Present
In significant amount
Present
NA
Low
NA
Less
NA
Don’t occur
not present
No difficulties
Difficult, problems/issues
Expensive
Relatively Small
Cheap
Long
Easy and Inexpensive
Difficult and Expensive
Required
Not Required
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Advantages of DC
Transmission

Only two conductors are required for DC transmission
system. It is further possible to use only one conductor of
DC transmission system if the earth is utilized as the
return path of the system.

The potential stress on the insulator of the DC
transmission system is about 70% of the equivalent
voltage AC transmission system. Hence, DC transmission
systems have reduced insulation costs.

Inductance, capacitance, phase displacement and surge
problems can be eliminated in DC system.
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Advantages of AC System

The alternating voltages can easily be stepped up and
down, which is not possible in DC transmission system.

Maintenance of AC substation is quite easy and
economical compared to DC.

The transforming of power in AC electrical substation is
much easier than motor-generator sets in a DC system.
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Disadvantages of AC System

The volume of conductor required in AC systems is much higher
compared to DC systems.

The reactance of the line affects the voltage regulation of the electrical
power transmission system.

Problems of skin effects and proximity effects only found in AC systems.

AC transmission systems are more likely to be affected by corona
discharge than a DC transmission system.

Construction of AC electrical power transmission network is more
complex than DC systems.

Proper synchronizing is required before interconnecting two or more
transmission lines together, synchronizing can totally be omitted in DC
transmission system.
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