In order to calculate the performance of a system under load conditions or upon
the occurrence of a fault, the one -line diagrm is used to draw the single- phase
or per-phase equivalent circuit of the system. Figure 1.26 combines the equivalent
circuits (yet to be developed) for the various components shown in Fig. 1 .25
to form the per-phase impedance diagram of the system. If a load study is to
bemade, the lagging loads A and B are represented by resistanc and inductive
reactancein series. The impedance diagram does not include the current-limiting
impedances shown in the one-line diagram between the neutrals of the'
generators and ground because no current flows in the ground under balanced
conditions and the neutrals of the generators are at the potential of the neutral
of the system. Since the shunt current of a transformer is usually insignificant
compared with the full-load current , the shunt admittance is usually omitted in
the equivalent circuit of the transformer.