Transmission Line
Introduction:
A transmission line is like a power highway for electricity to travel from a power plant to
faraway places where people use it. It’s made of special wires called ACSR, which stands for
aluminum conductor steel reinforced. The steel makes the wires strong. Most of the wires are
aluminum because it’s a good conductor and weighs less than copper. This helps electricity travel efficiently and safely. We’ll also talk about different types of transmission lines, their
characteristics, and how we model them in electrical engineering.
What is a Transmission Line:
Transmission lines are like electrical paths that help signals move from one place to another. They act as a connection between where the signal starts (transmitter) and where it ends (receiver). These lines enable electrical impulses to travel between two wires, and there’s usually something like air in between them.
Classification of Transmission Line:
Transmission lines are usually sorted based on how far they go, but in real situations, we
also look at frequency. So, we combine the length and frequency of a transmission line to classify them into three types.
Short transmission line.
Medium transmission line.
Long transmission line.
1. Short Transmission Line:
If you multiply the length and the frequency of a transmission line and get a number less
than 4000, we call it a short transmission line. For example, if the frequency is 50 Hz, the length of this line would be less than 80 km.
2. Medium Transmission Line:
If you multiply the length and the frequency of a transmission line and get a number
between 4000 and 10000, we call it a medium transmission line. For instance, at a frequency of 50 Hz, the length of this line would be somewhere between 80 km to 200 km.
3. Long Transmission Line:
If you multiply the length and the frequency of a transmission line and get a number greater
than 10000, we call it a long transmission line. For example, at a frequency of 50 Hz, the length of this line would be more than 200 km.
Modelling of the Transmission Line:
We can represent the ACSR conductor in a transmission line by using electrical values
like resistance, inductance, capacitance, and conductance. In simpler terms, we create a model that captures how the ACSR conductor behaves electrically by looking at these specific
parameters.
1. Resistance (R) :
The resistance of a transmission line depends on three things: resistivity (how resistant
the material is), length of the line (how long it is), and cross-sectional area (how thick it is). You can calculate the resistance using the formula: R = resistivity × length / cross-sectional area.
2. Inductance (L) :
When electricity flows through a transmission line, it creates a magnetic field between the
wires. We can represent this by calling it the inductance of the transmission line (L). In simpler terms, it’s like a measure of how much magnetic influence is created by the current moving through the wires.
3. Capacitance (C) :
In a transmission line, the wires for each phase are kept apart by air, which acts as a dielectric (insulating material). When current flows through these wires, there’s a natural capacity for them to store electrical energy, and we call this capacitance (C). This capacitance becomes more noticeable in medium and long transmission lines but is usually not a big deal in short ones. Simply put, it’s like a measure of how much electrical energy can be stored between the wires due to the air in between them.
4. Conductance (G) :
The imperfections in insulators can cause a small amount of current to leak through. We
describe this leakage with the conductance parameter (G). In long transmission lines, this leakage effect is notable, but we usually ignore it when analyzing short and medium transmission lines.
So, conductance is like a measure of how much current “leaks” through the insulators, but we only worry about it in long lines.
By using these four parameters the transmission line can be modeled as shown below.
To understand and work with the transmission line, we use parameters like R (resistance),
L (inductance), G (conductance), and C (capacitance). However, directly analyzing the circuit with these parameters can be tricky. So, to simplify things, we make some basic assumptions and introduce ABCD or transmission line parameters. Before diving into these models, it’s essential to have a basic understanding of these parameters. In simpler terms, we use ABCD parameters to make it easier to study and work with transmission lines by simplifying their representation.
Transmission Line Parameters or ABCD Parameters:
If you have voltage (Vs) and current (IS) at the start of the transmission line (supplying
end) and voltage (VR) and current (IR) at the end of the line (receiving end), you can express the relationship between them using the ABCD or transmission line parameters. In simpler words, it’s a way to describe how the voltage and current change from the beginning to the end of the transmission line
Assumptions in Transmission Line Modelling:
When we model a transmission line, we make a couple of basic assumptions:
We assume the system is balanced, meaning everything is in equilibrium.
We consider the network as a lumped system, which means we treat the
resistance (R), inductance (L), capacitance (C), and conductance (G) as if they are
concentrated in tiny parts of the network, making the analysis simpler. In simpler
terms, we break down the complex network into smaller, more manageable pieces.
Modelling of Short Transmission Line:
When we model a short transmission line, we can simplify things by ignoring the effects
of capacitance and conductance since they are not significant for short lines. This allows us to
draw a simpler equivalent circuit. In simpler words, for short transmission lines, we don’t need to worry much about capacitance and conductance, so we can represent the line more straightforwardly
