X
c
is the reactance of the series capacitor
p is the maximum allowable reach for an under-reaching zone with respect to the
sub-harmonic swinging related to the resulting fundamental frequency reactance
the zone is not allowed to over-reach.
The degree of compensation C in figure
219 has to be interpreted as the relation
between series capacitor reactance X
C
and the total positive sequence reactance X1
to the driving source to the fault. If only the line reactance is used the degree of
compensation will be too high and the zone 1 reach unnecessary reduced. The
highest degree of compensation will occur at three phase fault and therefore the
calculation need only to be performed for three phase faults.
The compensation degree in earth return path is different than in phases. It is for
this reason possible to calculate a compensation degree separately for the phase-to-
phase and three-phase faults on one side and for the single phase-to-earth fault
loops on the other side. Different settings of the reach for the ph-ph faults and ph-E
loops makes it possible to minimise the necessary decrease of the reach for
different types of faults.
Section 8 1MRK 506 369-UEN B
Impedance protection
396 Line distance protection REL670 2.2 IEC
Application manual
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