Load Encroachment - ABB REL670 2.2 IEC Applications Manual

Section 8
Impedance protection
8.3.2.3
214
The infeed factor (I +I
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in source impedances at local and remote end.
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IEC09000247 V1 EN-US
Figure 109: Influence of fault current infeed from remote line end
The effect of fault current infeed from remote line end is one of the most driving
factors for justify complementary protection to distance protection.
When the line is heavily loaded, the distance protection at the exporting end will
have a tendency to overreach. To handle this phenomenon, the IED has an adaptive
built-in algorithm, which compensates the overreach tendency of zone 1, at the
exporting end. No settings are required for this function.

Load encroachment

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 to
the left in figure 110. The entrance of the load impedance inside the characteristic
is of course 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 of figure 110. The load encroachment algorithm will increase the
possibility to detect high fault resistances, especially for phase-to-earth faults at
remote line end. For example, for a given setting of the load angle ArgLd for Phase
selection with load encroachment, quadrilateral characteristic function
(FDPSPDIS), the resistive blinder for the zone measurement can be expanded
according to the figure 110
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
)/I can be very high, 10-20 depending on the differences
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given higher fault resistance coverage without risk for
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IEC09000247-1-en.vsd
Line distance protection REL670 2.2 IEC
Application manual
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M17048-589 v8