ABB RET670 - Page 199

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
62. The load encroachment algorithm increases 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
62 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 loaded medium long lines. For short lines the major concern is to get
sufficient fault resistance coverage and load encroachment is not a major problem. So,
for short lines, the load encroachment function could preferable be switched off.
The settings of the parameters for load encroachment are done in the Phase selection
with load enchroachment, quadrilateral characteristic (FDPSPDIS, 21) function.
R
X
Zm
RldRev
R
Zm
RLdFwd
ZL
ANSI05000495_2_en.vsd
Load impedance area in
forward direction
LdAngle
LdAngle
LdAngle
LdAngle
ANSI05000495 V2 EN
Figure 62: Load encroachment phenomena and shaped load encroachment
characteristic
Long transmission line application
For long transmission lines the margin to the load impedance that is, to avoid load
encroachment, will normally be a major concern. It is difficult to achieve high
sensitivity for line to ground-fault at remote end of a long lines when the line is heavy
loaded.
Definition of long lines with respect to the performance of distance protection can
generally be described as in table
52, long lines have SIR’s less than 0.5.
1MRK504116-UUS C Section 3
IED application
193
Application manual