Load Encroachment; Long Transmission Line Application - ABB RELION 670 Series Applications Manual

Section 7
Impedance protection
The effect of fault current infeed from remote end is one of the most driving factors to justify
complementary protection to distance protection.
7.1.2.4

Load encroachment

Sometimes 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 57. The entrance of the load
impedance inside the characteristic is 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 57.
The load encroachment algorithm increases 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
ARGLd for the load encroachment function, the resistive blinder for the zone
angle
measurement can be expanded according to the right in figure
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. 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 selection with
load enchroachment, quadrilateral characteristic (FDPSPDIS) function.
IEC05000495 V1 EN-US
Figure 57: Load encroachment phenomena and shaped load encroachment
7.1.2.5

Long transmission line application

For long transmission lines the margin to the load impedance that is, to avoid load
encroachment, will normally be a major concern. It is difficult to achieve high sensitivity for line
to earth-fault at remote end of a long lines when the line is heavy loaded.
148
X
Zm
Load impedance
area in forward
direction
characteristic
ARGLd
R
ARGLd
RLdRv
1MRK 504 152-UEN B
57 given higher fault resistance
Zm
ZL
ARGLd
R
ARGLd
RLdFw
en05000495.vsd
Application manual
SEMOD168232-97 v3
SEMOD168232-128 v3