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
47.
Table 58: 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
115).
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 meaning LoadEnchMode = Disabled.
This will increase the possibility to detect resistive close-in faults.
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
59.
Table 59: 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 increase the security but might also lower the dependability since the
1MRK504116-UUS C Section 3
IED application
267
Application manual
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