The transmission system is a link between power plant and load. In power system network , the transmission line cover large area. It is most important task to study and design the transmission network.
An overhead transmission line has three constants Resistance (R), Inductance (L), and Capacitance (C). These parameters are uniformly distribute over the entire length of the transmission line. The resistance and inductance form the series impedance. For single phase line, the capacitance presents between the conductors and present between the conductor and natural. For three-phase transmission line, the capacitance forms a shunt path throughout the length of the transmission line. Hence, the capacitance effects create more complication in the modeling and calculation of transmission line.
The overhead transmission line classify into three types according to the manner in which capacitance is taken into account.
- Short transmission line
- Medium transmission line
- Long transmission line
Short transmission line
When the length of the line is up to about 50 km, the line considers as a short transmission line. The line voltage is completely low <20 kV. The capacitance effect in short overhead line is neglected. Because due to the small length and lower voltage, the capacitance effect is very small. Hence, in short, overhead transmission line, the capacitance effect is neglect. Therefore, while designing, modeling and studying the performance of the short line, only resistance and inductance are taken into account.
Medium transmission line
When the length of the line between 50 km to 150 km and the line voltage is between 20 kV to 100 kV, this type of overhead line considere as a medium transmission line. In this type of line, the capacitance effect cannot be neglect. Hence, while studying the performance of the line, the capacitance effect must include.
According to the distribution of the effect of capacitance, the medium transmission line is further divides into three parts; End condenser method, Nominal T method, and Nominal PI method.
End condenser method
In this method, the capacitance of line is lumped and concentrated at the receiving end or load end of the transmission line.
Nominal T method
In this method, the capacitance of the line assume to be concentrate at the middle point of the line. Half of the resistance and inductance are lumped on either side.
Nominal PI method
In this method, the capacitance of each conductor divides into two halves. One half is lump at the sending end and the second half lump at the receiving end.
Long transmission line
When the length of the transmission line is more than 150 km, the line considers as a long transmission line. In this type of transmission line, the voltage is more than 100 kV. For the modeling and designing of the long overhead transmission line, the capacitance effect is taken as uniformly distribute over the entire length of the line. The rigorous method use to solve the mathematical model of a long transmission line.
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