a
A B
A L f
A A
V
I I
Z p Z R
I I
+
= = × + ×
EQUATION1274 V2 EN (Equation 296)
The infeed factor (I
A
+I
B
)/I
A
can be very high, 10-20 depending on the differences in
source impedances at local and remote end.
21
ZL
21
EsA
VA
VA
A
B
EsB
IA
IB
Rf
p*ZL
(1-p)*ZL
ZSA
ZSB
en05000217_ansi.vsd
ANSI05000217 V1 EN
Figure 173: Influence of fault current infeed from remote line end
The effect of fault current infeed from the remote line end is one of the most driving factors
to justify complementary protection for distance protection.
When the line is heavily loaded, the distance protection at the exporting end will have a
tendency to overreach. To handle this phenomenon, the IED has an adaptive built-in
algorithm, which compensates the overreach tendency of zone 1 at the exporting end. No
settings are required for this feature.
8.12.2.3 Load encroachment
In some cases the measured load impedance might enter the set zone characteristic
without any fault on the protected line. This phenomenon is called load encroachment and
it might occur when an external fault is cleared and high emergency load is transferred
onto the protected line. The effect of load encroachment is illustrated on the left in figure
174. A load impedance within the characteristic would cause an unwanted trip. The
traditional way of avoiding this situation is to set the distance zone resistive reach with a
security margin to the minimum load impedance. The drawback with this approach is that
the sensitivity of the protection to detect resistive faults is reduced.
The IED has a built in feature which shapes the characteristic according to the
characteristic shown in figure
174. The load encroachment algorithm will increase the
possibility to detect high fault resistances, especially for phase-to-ground faults at remote
line end. For example, for a given setting of the load angle LdAngle, the resistive blinder
for the zone measurement can be set according to figure 174 affording higher fault
Section 8 1MRK 506 369-UUS -
Impedance protection
360 Line distance protection REL670 2.2 ANSI
Application manual
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