ABB RET670 - Page 327

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 141: 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.
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
49.
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
49. 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
49 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
1MRK504116-UUS C Section 3
IED application
321
Application manual