The transmission line having a length of more than 250km is considered a long transmission line. The line parameters are distributed over the entire length of a transmission line.
The equivalent circuit of a long transmission line is as shown below figure.
The entire length of a line is divided into several parts (n). Each part has a line constant (1/n) times the total line.
While modeling of long transmission line, we assume that the line constants are uniformly distributed over the line. And the resistance (R) and reactance (X) are series of lines.
Capacitance susceptance (B) and leakage conductance (G) are shunt elements of the line. The leakage current passes through the shunt admittance is maximum at the sending end side. And it decreases continuously towards the receiving end. At receiving end, the leakage current is zero.
Rigorous Method (Exact Solution of Long Transmission Line)
The exact solution of a long transmission line is given by the rigorous method. The equivalent circuit diagram of a small part of a transmission line is shown in the below figure.
Consider the length of this small part of the line is dx. And it is at X distance from the receiving end.
V = voltage at the end of element towards receiving end
V+dV = voltage at the end of element towards sending end
I = leaving current from the element dx
I+dI = current entering from the element dx
z = series impedance of line per unit length
y = shunt admittance of line per unit length
For small part dx;
Series impedance = zdx
Shunt admittance = ydx
In the direction of increasing x, rise in voltage over the element is;
Similarly, current entering to the element is I+dI and leaving from the element is I.
Current drawn by element is;
Differentiating equation-1 with respect to x.
The solution of this differential equation is;
Differentiate above equation with respect to x. Therefore;
Equation-3 and 4 gives the value of voltage and current in the form of unknown constants k1 and k2.
By applying end conditions; we can find the values for constants k1 and k2.
At receiving end;
Now, put these values in equation-3;
We have values of both unknowns. So, we can find the equations for voltage and current.
The entire length of transmission line is L. Therefore, if we consider the entire transmission line, we need to put;
ZC = surge impedance or characteristic impedance
ϒ = propagation constant
Therefore, the equation of long transmission line is;
Compare these equations with equations of ABCD parameters.
Equivalent Network of Long Transmission Line
The long transmission line network can be converted into two types of equivalent network;
- Equivalent-T network
- Equivalent-π network
Let’s derive equations for both types of networks;
Consider the circuit diagram similar to the Nominal-T method of a medium transmission line. Here, Z and Y are replaced by Z’ and Y’ respectively.
From the above figure;
Apply KVL to the loop;
Compare above equations with the equations of ABCD parameters;
These ABCD parameters are the same as the ABCD parameter of the Nominal-T Method.
From the exact solution of long transmission line by Rigorous method;
Now, compare the equation of equivalent-T method with the equations of rigorous method;
Find Shunt Branch of Equivalent-T Method
Multiply and divide by ϒ L;
Find Series Impedance of the Equivalent-T Method
Now, put the value of Y’ from above equation;
The series branch of equivalent T circuit multiply by the above factor.
The circuit diagram of equivalent-π method is as shown in below figure.
Sending end voltage;
Sending end current;
Compare above equation with the equation of ABCD parameters;
These ABCD parameters are same as the ABCD parameter of Nominal-π Method.
Compare above equations with the equations derived from the rigorous method;
Find Series Impedance of Equivalent-π Method.
Find Shunt Admittance of Equivalent-π Method.
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