High voltage power transmission lines and cables are represented depends on their length and the accuracy required. With increasing length of lines the shunt capacitance is added to equivalent circuit to form either Pi or T network. The T representation tends to be in more general use but there is little difference in accuracy between the two. For more information you can refer to capture 10 of "electrical transmission and distribution reference book" by Westinghouse or below descriptions.
The network must be reduced to a simple equivalent from which the constants for plotting the circle diagrams can be obtained. The simple equivalent can be expressed as a T or a Pi circuit or by giving the coefficients of the current and voltage equations, called the ABCD constants.
Comparison of the Equivalent Pi vs. ABCD Constants
The choice of the use of the equivalent pi vs. ABCD const. ants in calculating transmission-line constants is largely a matter of personal preference. However, each offers certain advantages over the other. When the network calculator is to be used, it is necessary to set up an actual circuit in the form of the equivalent pi. The equivalent pi affords a better physical picture of transmission-line performance and makes the comparison between long and short lines and the effect of charging current easier to visualize.
On the other hand, when a problem is to be solved analytically, the use of ABCD constants has a definite advantage over the equivalent pi because of the availability of the independent check: AD-CD= 1. This is particularly desirable when other circuits are to be combined with the transmission line circuit.
The equivalent pi or ABCD constants can be used to represent any line, section of line, or combination of lines and connected equipment. Either one represents accurately all conditions at the two terminals of the system. The equivalent circuit or ABCD constants being considered here pertains only to a single line or line section. The general equivalent circuit and general ABCD constants, if so desired, can be determined by the combination of the equivalent circuits for the rest of the system as discussed in Chapter 10.