ABB RET670 - Page 328

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 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
Figure 142: Load encroachment phenomena and shaped load encroachment
characteristic defined in Phase selection and load encroachment
function (FRPSPDIS, 21)
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
47.
Table 72: 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
Section 3 1MRK504116-UUS C
IED application
322
Application manual