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 problem for short line applications.
8.3.2.5 Long transmission line application
M17048-516 v5
For long transmission lines, the margin to the load impedance, that is, to avoid load
encroachment, will normally be a major concern. It is well known that it is difficult
to achieve high sensitivity for phase-to-earth fault at remote line end of long lines
when the line is heavy loaded.
What can be recognized as long lines with respect to the performance of distance
protection can generally be described as in table 20, long lines have Source
impedance ratio (SIR’s) less than 0.5.
Table 20: Definition of long and very long lines
Line category
Un Un
110 kV 500 kV
Long lines 77 km - 99 km 350 km - 450 km
Very long lines > 99 km > 450 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 at the same time as the security is
improved (risk for unwanted trip due to load encroachment is eliminated), see
figure
111.
Section 8 1MRK 506 369-UEN B
Impedance protection
216 Line distance protection REL670 2.2 IEC
Application manual
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