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2.10.1 Principle of distance measurement
(i) Phase-to-phase fault
The distance protection function used to detect phase-to-phase faults measures the impedance
from the relay to the fault point using delta voltage and current. The positive-sequence
impedance is used to represent the line impedance.
Figure 2.10-1 provides an illustration of the power system for the condition of a phase-to-
phase fault. The impedance from the relay to the fault is the same in phase-B and phase-C,
the self-impedance is Zs and the mutual impedance between phases is Zm. If the voltages and
currents of phase-B and phase-C are V
b
, V
c
, I
b
and I
c
and the fault point voltage is VF, then
V
b
and V
c
are given by the following equations.
From equations (2.10-1) and (2.10-2), the following equation is obtained.
where,
Zs: Self impedance
Zm: Mutual impedance
Since the effect of the phase-A current is small, and hence negligible in equation (2.10-3),
there is no need to introduce it in equations (2.10-1) and (2.10-2).
When each phase of the line is symmetric with the other, the positive-sequence and zero-
sequence impedances are Z
1
and Z
0
according to the method of symmetrical components. Z
1
and Z
0
are defined by the following equations, using the self-impedance Zs and the mutual
impedance Zm:
where,
Z
1
: Positive-sequence impedance
Z
0
: Zero-sequence impedance
Equation (2.10-3) can be rewritten as follows:
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