ABB RET670 Applications Manual page 294

Section 3
IED application
288
The effect of fault current infeed from remote end is one of the most driving factors for
justify complementary protection to distance protection.
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 129.
The entrance of the load impedance inside the characteristic is of cause 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 4. The load encroachment algorithm will increase 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 129 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. ZMMPDIS (21) 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).
1MRK504116-UUS C
Application manual