ABB Relion 670 series, Руководство по применению, Страница: 231 / 760

Table 20: Definition of short and very short line
Line category Un Un
110 kV 500 kV
Very short line 1.1-5.5 km 5-25 km
Short line 5-11 km 25-50 km
The use of load encroachment algorithm in Full-scheme distance protection, mho
characteristic function (ZMHPDIS) improves the possibility to detect high resistive faults
without conflict with the load impedance (see to the right of figure 139 ).
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 = Off. This will increase
the possibility to detect resistive close-in faults.
7.4.2.6 Long transmission line application
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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-earth 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 21 .
Table 21: Definition of 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 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 139 ).
It is recommended to use at least one of the load discrimination functions for long heavy
loaded transmission lines.
7.4.2.7 Parallel line application with mutual coupling
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General
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Introduction of parallel lines in the network is increasing due to difficulties to get necessary
area for new lines.
Parallel lines introduce an error in the measurement due to the mutual coupling between the
lines. The lines need not to be of the same voltage in order to experience mutual coupling, and
some coupling exists even for lines that are separated by 100 meters or more. The reason to
the introduced error in measuring due to mutual coupling is the zero sequence voltage
inversion that occurs.
1MRK 505 343-UEN B Section 7
Impedance protection
225
Application manual