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
19.
Table 21: Definition of short and very short line
Line category
Vn Vn
110 kV 500 kV
Very short line 0.75–3.6 miles 3–15 miles
Short line 4–7 miles 15–30 miles
The use of load encroachment algorithm in Full-scheme distance protection, mho
characteristic function (ZMHPDIS, 21) improves the possibility to detect high resistive
faults without conflict with the load impedance (see to the right of figure
126).
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 not a problem for short line applications so the load
encroachment function could be switched off meaning LoadEnchMode = Disabled. This
will increase the possibility to detect resistive close-in faults.
8.4.2.6 Long transmission line application
For long transmission lines the load encroachment will normally be a major concern. It is
well known that it is difficult to achieve high sensitivity for phase-to-ground fault at
remote end of a long line when the line is heavily loaded.
What can be recognized as long lines with respect to the performance of distance
protection is noted in table
22.
Table 22: Definition of long lines
Line category
Vn Vn
110 kV 500 kV
Long lines 77 km - 99 km 350 km - 450 km
Very long lines > 99 km > 450 km
The possibility to use the binary information from the 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). The possibility
to also use the blinder together with the load encroachment algorithm will considerably
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 259
Application manual
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