Load Encroachment - ABB RELION RET670 Applications Manual

Section 8
Impedance protection
8.13.2.3
400
V
A
Es
A
A
Z
SA
ANSI05000217 V1 EN-US
Figure 189: 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 to 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
and reduces the underreach at the importing 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 in the left part of
figure 190. 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 function which shapes the characteristic according to the right
part of figure 190. 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 expanded according to the right part of the figure 190,
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 heavily 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 loaded medium long lines. For short lines, the major concern is to get
sufficient fault resistance coverage. Load encroachment is not a major problem.
p*ZL
(1-p)*ZL
I
A
ZL
21
R
f
1MRK 504 163-UUS A
VA
Es
B
I
Z
B
SB
21
en05000217_ansi.vsd
GUID-D846C3D9-4C79-40B5-9ED2-23934214AADB v6
Transformer protection RET670 2.2 ANSI
Application manual
B