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. ZMMPDIS (21) 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 preferable be switched off.
The settings of the parameters for load encroachment are done in the Phase selection with
load enchroachment, quadrilateral characteristic (FDPSPDIS,21).
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 138: Load encroachment phenomena and shaped load encroachment
characteristic
8.5.2.5 Short line application
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 "Short line application".
Table 24: Definition of short and very short line
Line category
Vn Vn
110 kV 500 kV
Very short line
Short line
0.75-3.5 miles 3-15 miles
4-7 miles 15-30 miles
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-
Section 8 1MRK 506 369-UUS -
Impedance protection
284 Line distance protection REL670 2.2 ANSI
Application manual
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