Table 7: 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
For very short line applications the underreaching zone 1 cannot be used as the voltage
drop distribution through out the line will be too low causing risk for overreaching. If the
short lines have very high SIRs (>30) and are present with CVT for voltage measurement,
it is recommended to have CVT filter to reduce the voltage transients and hence the
tendency of overreaching for zone 1. CVT filter setting should be Enabled for such high
SIR lines.
Load encroachment is normally not a problem for short line applications so the load
encroachment function could be switched off meaning LoadEnchModex (where x is 1-5
depending on selected zone) = Disabled. This will increase the possibility to detect
resistive close-in faults.
6.1.2.4 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
8.
Table 8: 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 blinder might cut off
a larger part of the operating area of the circle (see to the right of figure
62).
It is recommended to use at least one of the load discrimination functions for long heavy
loaded transmission lines.
1MRK 506 334-UUS A Section 6
Impedance protection
119
Application manual
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