Line to Line Fault (LL):
Figure 11.7 shows a Line to Line Fault at F in a power system on phases b and c through a fault impedance Zf. The phases can always be relabelled, such that the fault is on phases b and c.
The currents and voltages at the fault can be expressed as
The symmetrical components of the fault currents are
from which we get
The symmetrical components of voltages at F under fault are
Writing the first two equations, we have
From which we get
Substituting Ib from Eq. (11.15) in Eq. (11.14), we get
Equations (11.11) and (11.16) suggest parallel connection of positive and negative sequence networks through a series impedance Zf as shown in Figs. 11.8a and b. Since Ia0 = 0 as per Eq. (11.12), the zero sequence network is unconnected.
In terms of the Thvenin equivalents, we get from Fig. 11.8b
From Eq. (11.15), we get
Knowing Ia1,we can calculate Va1 and Va2 from which voltages at the fault can be found.
If the fault occurs from loaded conditions, the positive sequence network can be modified on the lines.