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Electrical Engineering Power system Electrical Engineering Power system Transmission

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Electrical Engineering Power system Electrical Engineering Power system Transmission Powered By Docstoc
					        Electrical Engineering
              Power system
        Transmission Line Models:


 Overview

 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
 currents.

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
sending end
voltage and
current are                                                                                               (2.1)
then defined
in terms of the ABCD parameters as



                                                                                                           (2.2)

From (2.1) we
see that

                                                                                                           (2.3)



This implies that A is the ratio of sending end voltage to the open circuit receiving end voltage. This quantity
is dimension
less.
Similarly,
                                                                                                           (2.4)
                             Ω
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
also define

                                                                                                             (2.5)
                              mho



                                                                                                             (2.6)


The parameter D is dimension less.




                         Fig. 2.1 Two port representation of a transmission network.