Single-Line-to-Ground Fault

Let a 1LG fault has occurred at node k of a network. The faulted segment is then as shown in Fig. 8.2 where it is assumed that phase-a has touched the ground through an impedance Z_f . Since the system is unloaded before the occurrence of the fault we have

(8.1)

Fig. 8.2 Representation of 1LG fault.

Also the phase-a voltage at the fault point is given by

(8.2)

From (8.1) we can write

(8.3)

Solving (8.3) we get

(8.4)

This implies that the three sequence currents are in series for the 1LG fault. Let us denote the zero, positive and negative sequence Thevenin impedance at the faulted point as Z _kk0 , Z _kk1 and Z _kk2 respectively. Also since the Thevenin voltage at the faulted phase is V_f we get three sequence circuits that are similar to the ones shown in Fig. 7.7. We can then write

(8.5)

Then from (8.4) and (8.5) we can write

(8.6)

Again since

We get from (8.6)

(8.7)

The Thevenin equivalent of the sequence network is shown in Fig. 8.3.

Fig. 8.3 Thevenin equivalent of a 1LG fault.

Example 8.1