resistance coverage without risk for unwanted operation due to load encroachment.
Separate resistive blinder settings are available in forward and reverse direction.
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. See section
"Load
impedance limitation, without load encroachment function".
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 174: Load encroachment phenomena and shaped load encroachment
characteristic
[1]
8.12.2.4 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 such a common problem. 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.
[1] RLdRv=RLdRvFactor*RLdFw
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 361
Application manual
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