CLASSIFICATION OF TRANSMISSION LINE

Classification of Transmission Line
There are three major approaches in analyzing transmission
lines in a power system, the Short, Medium and
Long transmission line analysis.

These three classifications affects the analysis since certain electrical parameters are considered which differ from one to the other. According to the book of William Stevenson Jr. entitled "Elements of Power System Analysis", in an open wire 60hz line, it is said that;

Short Lines is less than about 80 kilometers (50miles).
Medium Lines are between 80 kilometers (50 miles) and 240 kilometers(150 miles)
Long Lines are longer than 240 kilometers(150 miles).

In an overhead lines that is classified as short, the major line parameters that are considered is the series resistance and the series inductance of the line. The power dissipated in this two parameters practically also represents the line loss of the conductor. Shunt capacitance is not considered in the analysis of short lines since the absence of it creates a little loss of accuracy.

SHORT LINE ANALYSIS

Medium line on the other hand considers the shunt capacitance in the analysis. Two types of analysis are usually done with medium length transmission lines. The pi network and the T network. In a pi network, the line impedance R & L are lumped in the middle of the line which represents that it is uniformly distributed along the line. With it the shunt capacitance are divided in to two, the first half is found in the sending end while the other is at the receiving end. While in the T network, the opposite is usually done. In a T network, the shunt capacitance are lumped in the middle of the line and the total line impedance is equally divided into two along the each side of the network.

PI NETWORK FOR MEDIUM LINE ANALYSIS

T NETWORK FOR MEDIUM LINE ANALYSIS

Lastly, when the transmission line is over 240 kilometers and that the analysis now requires a high degree of accuracy, long line analysis is practically used. However, we shall not dwell so much with these analysis and the primary objective of this discussion is to refresh your ideas about transmission lines.

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