99000202.vsd
p
100
80
60
40
20
0
100 %80604020
C
%
IEC99000202 V1 EN
Figure 103: Reduced reach due to the expected sub-harmonic oscillations at different
degrees of compensation
c
l
X
c degreeof compensation
X
æ ö
=
ç ÷
ç ÷
è ø
EQUATION1894 V1 EN (Equation 78)
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 103 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 ground 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-ground fault loops on the other
side. Different settings of the reach for the ph-ph faults and ph-G loops makes it possible
to minimise the necessary decrease of the reach for different types of faults.
Section 8 1MRK 506 369-UUS -
Impedance protection
206 Line distance protection REL670 2.2 ANSI
Application manual
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