2.3 Medium Line Approximation

• Normal -π Representation
• Normal- T Representation

Medium transmission lines are modeled with lumped shunt admittance. There are two different representations - nominal- π and nominal-T depending on the nature of the network. These two are discussed here one by one.

Nominal- π Representation

In this representation the lumped series impedance is placed in the middle while the shunt admittance is divided into two equal parts and placed at the two ends. The nominal- π representation is shown in Fig. 2.3. This representation is used for load flow studies, as we shall see later. Also a long transmission line can be modeled as an equivalent π -network for load flow studies.

Fig. 2.3 Nominal- p representation.

Let us define three currents I₁ , I₂ and I₃ as indicated in Fig. 2.3. Applying KCL at nodes M and N we get

(2.10)

(2.11)

Substituting (2.11) in (2.10) we get

(2.12)

Therefore from (2.11) and (2.12) we get the following ABCD parameters of the nominal- p representation

	(2.13)
	(2.14)
	(2.15)