Transmission Line Models:
As we have discussed earlier in Chapter 1 that the transmission line parameters include series resistance
and inductance and shunt capacitance. In this chapter we shall discuss the various models of the line. The
line models are classified by their length. These classifications are
Short line approximation for lines that are less than 80 km long.
Medium line approximation for lines whose lengths are between 80 km to 250 km.
Long line model for lines that are longer than 250 km.
These models will be discussed in this chapter. However before that let us introduce the ABCD
parameters that are used for relating the sending end voltage and current to the receiving end voltage and
Section I: ABCD Parameters
Consider the power system shown in Fig. 2.1. In this the sending and receiving end voltages are denoted by
VS and VR respectively. Also the currents IS and IR are entering and leaving the network respectively. The
current are (2.1)
in terms of the ABCD parameters as
From (2.1) we
This implies that A is the ratio of sending end voltage to the open circuit receiving end voltage. This quantity
i.e., B , given in Ohm, is the ratio of sending end voltage and short circuit receiving end current. In a similar
way we can
The parameter D is dimension less.
Fig. 2.1 Two port representation of a transmission network.