ABB RET670 - Page 271

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 for the mho circle is illustrated to the
left in 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.
jX
R
Load
Load
Load
Load
jX
R
Load
Load
Load
Load
No
operation
en06000403.vsd
IEC06000403 V1 EN
Figure 115: Load encroachment phenomena and shaped load encroachment
characteristic
The Faulty phase identification with load encroachment for mho (FMPSPDIS, 21)
function shapes the characteristic according to the diagram on the right in figure 115.
The load encroachment algorithm will increase the possibility to detect high fault
resistances, especially for phase-to-ground faults at remote line end. For example, for a
given setting of the load angle LdAngle (see figure
116) for the Faulty phase
identification with load encroachment for mho function (FMPSPDIS, 21), the zone
reach can be expanded according to the diagram on the right in figure 115 given higher
fault resistance coverage without risk for unwanted operation due to load
encroachment. The part of the load encroachment sector that comes inside the mho
circle will not cause a trip if FMPSPDIS (21) is activated for the zone measurement.
This is valid in both directions.
1MRK504116-UUS C Section 3
IED application
265
Application manual