Friday, 15 July 2016

Basis of Classification of Transmission Line

A Transmission Line classification based on its length are Short, Medium and Long lines. The length of line is calculated between the two substations of concern.
  • If the length of line is less than 80 km then it is said to be Short Transmission Line.
  • The length of Medium Transmission Line varies from 80 to 160 km. Therefore a transmission line of length 120 km will said to be a Medium Transmission Line.
  • If the length of line is more than 160 km then line is said Long Line.
You will be surprised that where from these magical numbers like 80 and 160 came from. Don’t worry, I will be explaining.

Basis of Classification of Transmission Line

Well, we know that current and voltage propagate in a transmission line with a speed equal to the speed of light i.e. 3×108m/sec = 3×105km/sec. The frequency (f) of voltage/current is 50Hz, therefore wavelength (λ) can be calculated as below.
 f.λ = v
⇒ λ = 3×105 / 50 = 6000 km
Thus 80 km is around (1/75) times of wavelength of current/voltage wave which is very very small when compared with 6000 km. Refer the figure below.

Up to 160 km length of line, current/voltage waveform can be approximated to OZ. OZ is so small that it can be approximated to straight line for all practical purposes. Therefore the response of transmission line for a length up to 160 km is linear and hence the equivalent circuit of line can be approximated to linear circuit. This is why, for the analysis of short and Medium transmission line we consider the circuit to be liner electrical circuit.
But if the length of line is more than 160 km say 400 km then the waveform of current/voltage can not be considered liner. This is the reason, we can not consider long transmission line equivalent circuit to be liner electrical circuit. Since the long transmission line equivalent circuit is not liner, therefore we need to use integration for the analysis of long line. This is the basis for classification of transmission line.
Next, the question arises how do we consider resistance, inductance and capacitance of Medium, Short Line and Long Line?
Three electrical parameters i.e. resistance, inductance and capacitance are uniformly distributed over the entire length of the line. For short transmission line, resistance and inductance of line is considered lumped. The effect of shunt capacitance is neglected for the analysis.
For Medium Transmission Line, resistance, inductance and capacitance are considered lumped but there exists different model line T Model and Pi Model. You are requested to read T Model and Pi Model.
In long Transmission line, none of the parameter can be considered lumped rather we need to use rigorous calculation using integration to analyze the system.

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