Line-To-Line Fault

In Figure 7.14, E = e.m.f. per phase and the R phase is again taken as the reference phasor. In this case, IR = 0, Iy = —IB, and Vy = VB.

Interconnection of positive-, negative-, and zero-sequence networks for single-line-to-earth faults

Figure 7.13 Interconnection of positive-, negative-, and zero-sequence networks for single-line-to-earth faults

Line-to-line fault on phases Y and B

Figure 7.14 Line-to-line fault on phases Y and B

From equation (7.4), and

As VY = VB (but not equal to zero) hence

This can be represented by the equivalent circuit in Figure 7.15, in which, of course, there is no zero-sequence network. If the connection between the two lines has an impedance Zf (the fault impedance), this is connected in series in the equivalent circuit.

Interconnection of sequence networks for a line-to-line fault (including fault impedance Zf, if present)

Figure 7.15 Interconnection of sequence networks for a line-to-line fault (including fault impedance Zf, if present)

Line-To-Line-To-Earth Fault (Figure 7.16)

and

and Line-to-line-to-earth fault

Figure 7.16 Line-to-line-to-earth fault

Interconnection of sequence networks-double line-to-earth fault

Figure 7.17 Interconnection of sequence networks-double line-to-earth fault

Hence,

and

These can be represented by the equivalent circuit as shown in Figure 7.17.

The inclusion of impedances in the earth path, such as the star-point-to earth connection in a generator or transformer, modifies the sequence diagrams. For a line-to- earth fault an impedance Zg in the earth path is represented by an impedance of 3Zg in the zero-sequence network. Zg can include the impedance of the fault itself, usually the resistance of the arc. As Ij = I2 = I0 and 3Ij flows through Zg in the physical system, it is necessary to use 3Zg to obtain the required effect. Hence,

Again, for a double-line-to-earth fault an impedance 3Zg is connected as shown in Figure 7.18. Zg includes both machine neutral impedances and fault impedances.

The phase shift introduced by star-delta transformers has no effect on the magnitude of the fault currents, although it will affect the voltages at various points. If the positive-sequence voltages and currents are advanced by a certain angle then the negative-sequence quantities are retarded by the same angle for a given connection.

Modification of network in Figure 7.17 to account for neutral impedance Zg

Figure 7.18 Modification of network in Figure 7.17 to account for neutral impedance Zg

 
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