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R
X
Zm
Load impedance
area in forward
direction
RLdRv
R
Zm
ARGLd
ARGLd
ARGLd
ARGLd
RLdFw
ZL
en05000495.vsd
IEC05000495 V1 EN-US
Figure 152: Load encroachment phenomena and shaped load encroachment
characteristic
7.5.2.5
Short line application
SEMOD154680-105 v2
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 23: Definition of short and very short line
Line category
Un
Un
110 kV
500 kV
Very short line
Short line
1.1-5.5 km
5-25 km
5.5-11 km
25-50 km
The possibility in IED to set resistive and reactive reach independent for positive and zero
sequence fault loops and individual fault resistance settings for phase-to-phase and phase-to-
earth fault together with load encroachment algorithm improves the possibility to detect high
resistive faults without conflict with the load impedance, see figure
.
For very short line applications the underreaching zone 1 can not be used due to that the
voltage drop distribution through out the line will be too low causing risk for overreaching.
Load encroachment is normally no problems for short line applications so the load
encroachment function could be switched off (
OperationLdCmp = Off). This will increase the
possibility to detect resistive close-in faults.
7.5.2.6
Long transmission line application
SEMOD154680-127 v2
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
phase-to-earth fault at remote end of a long lines when the line is heavy loaded.
The definition of long lines with respect to the performance of distance protection is noted in
.
1MRK 505 343-UEN B
Section 7
Impedance protection
247
Application manual
Summary of Contents for Relion 670 series
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