The Short Line (up to 80 km, 50 miles)

The equivalent circuit is shown in Figure 3.26 and it will be noticed that both shunt capacitance and leakage resistance have been neglected. The four-terminal network constants are (see Section 2.7):

The regulation of a circuit is defined as:

It should be noted that if I leads VR in phase, that is a capacitive load, then VR > VS, as shown in Figure 3.27.

Figure 3.25 Distributed constant representation of a line: L = inductance of line to neutral per unit length; r = a.c. resistance per unit length; C = capacitance line to neutral per unit length; R = leakage resistance per unit length

Figure 3.26 Equivalent circuit of a short line - representation under balanced three phase conditions

Medium-Length Lines (up to 240 km, 150 miles)

Owing to the increased length, the shunt capacitance is now included to form either a p or a T network. The circuits are shown in Figure 3.28. Of these two versions the p representation tends to be in more general use but there is little difference in accuracy between the two. For the p network:

from which Vs and Is are obtained in terms of VR and IR giving the following constants:

Figure 3.27 Phasor diagram for short line on leading power factor load

Figure 3.28 (a) Medium-length line - p equivalent circuit, (b) Medium-length line - T equivalent circuit

Similarly, for the T network: giving