sufficient fault resistance coverage and load encroachment is not a major problem. So,
for short lines, the load encroachment function could preferably be switched off. See
section
"Load impedance limitation, without load encroachment function"
.
The settings of the parameters for load encroachment are done in (21), FRPSPDIS (21)
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-US
Figure 153:
Load encroachment phenomena and shaped load encroachment
characteristic defined in Phase selection and load encroachment
function (FRPSPDIS, 21)
8.9.2.4
Short line application
GUID-504F03E4-37CA-4CCF-9DBB-82B8F1C3B3DD v1
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
Table 30:
Typical length of short and very short line
Line category
Vn
Vn
110 kV
500 kV
Very short line
0.75 -3.5mile
3-15 miles
Short line
4-7 miles
15-30 miles
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI
327
Application manual
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