a
A B
A L f
A A
V
I I
Z p Z R
I I
+
= = × + ×
EQUATION1274 V2 EN (Equation 194)
The infeed factor (IA+IB)/IA can be very high, 10-20 depending on the differences in
source impedances at local and remote end.
Z <
ZL
Z <
EsA
VA
VA
A
B
EsB
IA
IB
Rf
p*ZL
(1-p)*ZL
ZSA
ZSB
en05000217.vsd
IEC05000217 V1 EN
Figure 137: Influence of fault infeed from remote end.
The effect of fault current infeed from remote end is one of the most driving factors for
justify complementary protection to distance protection.
8.5.2.4 Load encroachment
In some cases the load impedance might enter the zone characteristic without any fault on
the protected line. The phenomenon is called load encroachment and it might occur when
an external fault is cleared and high emergency load is transferred on the protected line.
The effect of load encroachment is illustrated to the left in figure
138. The entrance of the
load impedance inside the characteristic is of cause not allowed and the way to handle this
with conventional distance protection is to consider this with the settings that is, to have
a security margin between the distance zone and the minimum load impedance. This has
the drawback that it will reduce the sensitivity of the protection that is, the ability to detect
resistive faults.
The IED has a built in function which shapes the characteristic according to the right
figure 4. The load encroachment algorithm will increase the possibility to detect high fault
resistances, especially for line to ground faults at remote end. For example for a given
setting of the load angle LdAngle for the load encroachment function, the resistive blinder
for the zone measurement can be expanded according to the right in figure
138 given
higher fault resistance coverage without risk for unwanted operation due to load
encroachment. This is valid in both directions.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 283
Application manual
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