ABB RET670 Applications Manual page 170

Section 3
IED application
164
The infeed factor (I +I
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source impedances at local and remote end.
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Figure 48: 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 49.
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.
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
figure 115. 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.
)/I can be very high, 10-20 depending on the differences in
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Application manual
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