Fault Subsequent Clearance by Circuit Breaker

Let us consider a single machine tied to infinite bus through two parallel lines as shown below in figure.

Let a fault occurs in the middle of one of the line (say X2 line).

Using delta-star transformation circuit of above figure can be reduced as shown below in figure.

Using star-delta transformation above figure can be reduced to below figure as shown below.

The power angle curve for the three different cases, i.e., before fault (prefault), during fault and post fault are respectively shown below in figure.