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7.12.2.3
Load encroachment
GUID-6785BF05-2775-4422-8077-A663D01C6C07 v4
In some cases the measured load impedance might enter the set zone characteristic without
any fault on the protected line. This phenomenon is called load encroachment and it might
occur when an external fault is cleared and high emergency load is transferred onto the
protected line. The effect of load encroachment is illustrated on the left in figure
. The
entrance of the load impedance inside the characteristic is of course not desirable and the way
to handle this with conventional distance protection is to consider this with the resistive reach
settings, that is, to have a security margin between the distance zone characteristic and the
minimum load impedance. Such a solution has the drawback that it will reduce the sensitivity
of the distance protection, that is, the ability to detect resistive faults.
The IED has a built in feature which shapes the characteristic according to the characteristic
. The load encroachment algorithm will increase the possibility to detect
high fault resistances, especially for phase-to-earth faults at the remote line end. For example,
for a given setting of the load angle
ArgLd, the resistive blinder for the zone measurement can
affording higher fault resistance coverage without risk for
unwanted operation due to load encroachment. Separate resistive blinder settings are
available in forward and reverse direction.
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. See section
""
.
R
X
Z1
Load impedance
area in forward
direction
RLdRv
R
Z1
ArgLd
RLdFw
IEC09000248-2-en.vsd
X
IEC09000248 V2 EN-US
Figure 188: Load encroachment phenomena and shaped load encroachment
characteristic
[1]
7.12.2.4
Short line application
GUID-331451C9-EA93-481A-BA2E-D729EDB98828 v3
In short line applications, the major concern is to get sufficient fault resistance coverage. Load
encroachment is not such a common problem. The line length that can be recognized as a
short line is not a fixed length; it depends on system parameters such as voltage and source
.
[1] RLdRv=RLdRvFactor*RLdFw
Section 7
1MRK 505 343-UEN B
Impedance protection
306
Application manual
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