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 heavily 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. Load encroachment is not a major problem. Nevertheless, always set
RLdFwd, RldRev and LdAngleaccording to the expected maximum load since these
settings are used internally in the function as reference points to improve the performance
of the phase selection.
R
X
Zm
RldRev
R
Zm
RLdFwd
ZL
ANSI05000495_2_en.vsd
Load impedance area in
forward direction
LdAngle
LdAngle
LdAngle
LdAngle
ANSI05000495 V2 EN
Figure 188: Load encroachment phenomena and shaped load encroachment
characteristic
8.13.2.4 Short line application
Transmission line lengths for protection application purposes are classified as short,
medium and long. The definition of short, medium and long lines is found in IEEE Std
C37.113-1999. The length classification is defined by the ratio of the source impedance at
the protected line’s terminal to the protected line’s impedance (SIR). SIR’s of about 4 or
greater generally define a short line. Medium lines are those with SIR’s greater than 0.5
and less than 4.
In short line applications, the major concern is to get sufficient fault resistance coverage.
Load encroachment is not so common. The line length that can be recognized as a short
line is not a fixed length; it depends on system parameters such as voltage and source
impedance, see table
19.
Section 8 1MRK 506 369-UUS -
Impedance protection
388 Line distance protection REL670 2.2 ANSI
Application manual
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