A medium transmission line is a type of overhead transmission line that is used to transmit electrical power over a distance of more than 80 km but less than 250 km. The parameters of resistance, inductance, and capacitance are distributed uniformly along the line, which means that the value of these parameters vary continuously along the length of the line.
For medium transmission lines, the charging current is significant and due to the length of the line, the shunt admittance plays a significant role in the calculation of the effective parameters of the line. Shunt admittance is the measure of the ability of the line to conduct current that is perpendicular to the main current flow. Because medium transmission lines are longer than short transmission lines, they have more capacitance and more shunt admittance, which affects the performance of the line.
To simplify the calculations and make the analysis of the line more manageable, the shunt admittance and series impedance are often considered as a lumped parameter of the medium transmission line. This means that the shunt admittance and series impedance are treated as a single value at a specific point on the line, rather than as a continuous distribution along the length of the line.
There are many localized capacitance models that are used to make approximate line performance calculations for medium transmission lines. The following models are commonly used:
- Nominal T model: This model is used to represent the capacitance of a medium transmission line by assuming that it is concentrated at two different points along the line, creating a T-shaped distribution of capacitance. The model is suitable for long lines with high inductance-capacitance ratios.
- Nominal π (pi) model: This model is used to represent the capacitance of a medium transmission line by assuming that it is concentrated at three different points along the line, creating a π-shaped distribution of capacitance. The model is suitable for short lines with low inductance-capacitance ratios.
It is important to note that each model has its own advantages and disadvantages and the choice of model will depend on the specific requirements of the transmission line and the goals of the analysis. Engineers will consider the actual placement of the conductors, the voltage level, the line length, and other factors while choosing the models.