8.1.2.4 Load encroachment
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
effect of load encroachment is illustrated to the left in figure
59. 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 59. 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 59 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 59: Load encroachment phenomena and shaped load encroachment
characteristic
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 161
Application manual
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