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As shown above, the positive-sequence impedance is used when determining settings for
phase faults.
Figure 2.10-1 Phase-to-phase fault
(ii) Single-phase-earth fault
Figure 2.10-2 shows the power system in the event of a single-phase-earth fault. It is very
difficult to measure precisely the distance to the fault point for a single-phase-earth fault. This
is because the impedance of the zero-sequence circuit including the earth return is generally
different from the positive-sequence impedance. Therefore, the faulted phase voltage is not
simply proportional to the faulted phase current.
Figure 2.10-2 Single-phase-earth fault
It is necessary to analyze the impedance seen by the relay in the event of a single-phase-
earth fault according to the method of symmetrical components. Figure 2.10-3 shows an
equivalent circuit for a single-phase-earth fault based on the method of symmetrical
components. If we define the positive-sequence, negative-sequence and zero-sequence voltages
as being V
1
F, V
2
F and V
0
F, then the voltage at the relaying point of each symmetrical circuit
will be given by the following equations based upon the assumption that the positive-sequence
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