ABB RED670 Handbuch Seite 288

Abschnitt 4
IED Anwendung
284
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 149. 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 i.e. 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 i.e. the ability to detect
resistive faults.
The REL 670 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 earth faults at remote end. For
example for a given setting of the load angle ARGLd for the load encroachment
function, the resistive blinder for the zone measurement can be expanded according
to the right in figure 149
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. 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 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 selector
function PHS.
X
Zm
Lastimpedanzbereich
in Vorwärtsrichtung
IEC05000495 V1 DE
Abb. 149: Load encroachment phenomena and shaped load encroachment
characteristic
given higher fault resistance coverage without risk for
ARGLd
R
ARGLd
1MRK 505 186-UDE B
Zm
ZL
ARGLd
ARGLd
RLdRv RLdFw
en05000495.vsd
Applikationshandbuch
R