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 ef
fect of load encroachment is illustrated to the left in figure
70.
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 70. 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 70 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
suf
ficient 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-US
Figure 70: Load encroachment phenomena and shaped load encroachment
characteristic
1MRK 504 163-UUS A Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 193
Application manual
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