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That is, the positive-sequence impedance Z
1
up to the fault point can be obtained from
the simple ratio of voltage V
a
to compensated current I
a
' according to equation (2.10-14).
Obtaining the compensated current according to equation (2.10-13) is called "zero-
sequence compensation." Note that in this implementation of zero-sequence compensation, the
compensation coefficient
and
are not real numbers, but complex numbers. The distance protection function has a
configuration that allows this compensation coefficient to be set as a complex number and
setting the coefficient correctly makes it possible to measure exactly the distance to the fault
point.
In equations (2.10-7) to (2.10-9), the fault resistance is ignored. Since the measurement
of the distance up to the fault point, based on equation (2.10-14) is carried out using the
reactance component, in principle there is no influence on the voltage component due to the
fault resistance. However, under real operating conditions, distance measurement errors are
introduced because of the fault resistance combined with the power flow or the current flowing
into the fault point from the remote terminal opposite to the relay location.
Figure 2.10-3 Equivalent circuit of single-phase earth fault
Positive-sequence circuit
Negative-sequence circuit
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