Medium Transmission Line
The transmission line having its effective length more than 80 km but less than 250 km is generally referred to as a medium transmission line. Due to the line length being considerably high, admittance Y of the network does play a role in calculating the effective circuit parameters, unlike in the case of short transmission lines. For this reason the modeling of a medium length transmission line is done using lumped shunt admittance along with the lumped impedance in series to the circuit.These lumped parameters of a medium length transmission line can be represented using three different models, namely-
- Nominal Π representation.
- Nominal T representation.
- End Condenser Method.
Nominal Π Representation of a Medium Transmission LineIn case of a nominal Π representation, the lumped series impedance is placed at the middle of the circuit where as the shunt admittances are at the ends. As we can see from the diagram of the Π network below, the total lumped shunt admittance is divided into 2 equal halves, and each half with value Y ⁄ 2 is placed at both the sending and the receiving end while the entire circuit impedance is between the two. The shape of the circuit so formed resembles that of a symbol Π, and for this reason it is known as the nominal Π representation of a medium transmission line. It is mainly used for determining the general circuit parameters and performing load flow analysis. As we can see here, VS and VR is the supply and receiving end voltages respectively, and Is is the current flowing through the supply end. IR is the current flowing through the receiving end of the circuit. I1 and I3 are the values of currents flowing through the admittances. And I2 is the current through the impedance Z
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Comparing equation (4) and (5) with the standard ABCD parameter equations We derive the parameters of a medium transmission line as: