The use of the load encroachment feature is essential for long heavy 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 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
.
The settings of the parameters for load encroachment are done in , FRPSPDIS function.
R
X
Z1
Load impedance
area in forward
direction
RLdRv
R
Z1
ArgLd
RLdFw
IEC09000248-2-en.vsd
X
IEC09000248 V2 EN-US
Figure 165: Load encroachment phenomena and shaped load encroachment
characteristic defined in Phase selection and load encroachment function
(FRPSPDIS)
7.9.2.4
Short line application
GUID-504F03E4-37CA-4CCF-9DBB-82B8F1C3B3DD v1
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 25: Typical length of short and very short line
Line category
Un
Un
110 kV
500 kV
Very short line
1.1-5.5 km
5-25 km
Short line
5.5-11 km
25-50 km
The IED's ability 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 the voltage
drop distribution throughout the line will be too low causing risk for overreaching.
Load encroachment is normally no problems for short line applications.
1MRK 505 343-UEN B
Section 7
Impedance protection
271
Application manual
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