8.13.2.3 Load encroachment
GUID-D846C3D9-4C79-40B5-9ED2-23934214AADB v6
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
186. 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 186. 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 be expanded according to the right
part of the figure
186, 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.
Nevertheless, always set RLdFw, RLdRv
[5]
and ArgLd according to the expected
maximum load since these settings are used internally in the function as reference
points to improve the performance of the phase selection.
R
Z1
ArgLd
RLdFw
IEC09000248-3-en.vsdx
X
RLdRv
[1]
IEC09000248 V3 EN-US
Figure 186: Load encroachment phenomena and shaped load encroachment
characteristic
[5] RLdRv=RLdRvFactor*RLdFw.
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 363
Application manual
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