Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
155
The earth fault resistance measured by the distance protection then results from the formula below (it is
assumed that I
1
and I
E
are in phase opposition):
with
R
RE
Resistance measured by the SIPROTEC distance protection
R
L1
Line resistance up to the fault location
R
Arc
Arc resistance
R
E
/R
L
Setting in the distance protection (address 1116 and 1118)
I
2
/I
1
Ratio between earth fault currents at the opposite end and the local end. For a correct R
setting of the distance zone, the most unfavourable case must be considered. This most un-
favourable case would be a maximum earth fault current at the opposite end and a minimum
earth fault current at the local end. Moreover, the currents considered are the r.m.s. values
without phase displacement. Where no information is available on the current ratio, a value
of approx. „3“ can be assumed. On radial feeders with negligible infeed from the opposite
end, this ratio is „0“.
R
TF
Effective tower footing resistance of the overhead line system. Where no information is avail-
able on the amount of tower footing resistance, a value of 3 Ω can be assumed for overhead
lines with earth wire (see also /5/).
The following recommended setting
applies for the resistance tolerance of distance zone Z1:
with
R
1E
Setting in the distance protection RE(Z1) Ø-E, address 1604
1.2 Safety margin 20%
The resistance R
L
of the line itself can be ignored with SIPROTEC 4 devices. It is taken into account by the
shape of the polygon, provided that the inclination angle of the polygon Distance Angle (address 1511) is
not set greater than the line angle Line Angle (address 1105).
Example
:
Arc length: 2 m
Minimum fault current: 1.0 kA
Effective tower footing resistance of the overhead line system: 3 Ω
with
I
2
/I
1
= 3
R
E
/R
L
= 0,6
Voltage transformer 110 kV / 0.1 kV
Current transformer 600 A / 5 A
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