With reference to figure 56, we can draw the equation for the bus voltage Va at left
side as:
( )A A L A B fV I p Z I I R= × × + + ×
EQUATION1273 V1 EN-US (Equation 25)
If we divide Va by IA we get Z present to the IED at A side
a
A B
A L f
A A
V
I I
Z p Z R
I I
+
= = × + ×
EQUATION1274 V2 EN-US (Equation 26)
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-US
Figure 56: Influence of fault infeed from remote end
The effect of fault current infeed from remote end is one of the most driving factors
to justify complementary protection to distance protection.
8.1.2.4 Load encroachment
SEMOD168232-97 v3
Sometimes 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
57. The entrance of the load impedance inside the characteristic is
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
57. The load encroachment algorithm increases the possibility to detect
high fault resistances, especially for line to earth faults at remote end. For example,
for a given setting of the load angle ARGLd for the load encroachment function, the
Section 8 1MRK 506 369-UEN B
Impedance protection
150 Line distance protection REL670 2.2 IEC
Application manual
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