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 150: Influence of fault current infeed from remote line end
The effect of fault current infeed from remote line end is one of the most driving factors
for justify complementary protection to 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 function.
8.9.2.3 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
151. The entrance of the
load impedance inside the characteristic is of course 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 of figure
151. 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 for Phase selection with load
encroachment, quadrilateral characteristic function (FRPSPDIS, 21), the resistive blinder
for the zone measurement can be expanded according to the figure
151 given higher fault
resistance coverage without risk for unwanted operation due to load encroachment. This
is valid in both directions.
The use of the load encroachment feature is essential for long heavy loaded lines, where
there might be a conflict between the necessary emergency load transfer and necessary
sensitivity of the distance protection. The function can also preferably be used on heavy
Section 8 1MRK 506 369-UUS -
Impedance protection
314 Line distance protection REL670 2.2 ANSI
Application manual
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