This section presents an easier method for derivation of the transmission loss formula. Fig. 2.1 represents two generating units 1 and 2 feeding a number of loads, totaling to IL through a large transmission network. Out of the many branches within the network, consider a branch (any three-phase transmission line) designated as b.
Fig 2.1 Network with two generators feeding a large number of loads, and any one branch b of network
Let the switch in Fig.2.1(a) be open. Then the total load current IL is supplied by generator 1 only. With generator 1 acting alone, the current in the line b is Ib1