Load encroachment is normally no problems for short line applications so the load
encroachment function could be switched off (
OperationLdCmp = Off
). This will
increase the possibility to detect resistive close-in faults.
8.5.2.6
Long transmission line application
SEMOD154680-127 v2
For long transmission lines the margin to the load impedance that is, to avoid load
encroachment, will normally be a major concern. It is difficult to achieve high
sensitivity for phase-to-ground fault at remote end of a long lines when the line is
heavy loaded.
The definition of long lines with respect to the performance of distance protection is
noted in table
Table 29:
Definition of long lines
Line category
Vn
Vn
110 kV
500 kV
Long lines
45-60 miles
200–250 miles
Very long lines
>60 miles
>250 miles
As mentioned in the previous chapter, the possibility in IED 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-ground 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).
8.5.2.7
Parallel line application with mutual coupling
SEMOD154680-148 v1
General
SEMOD154680-150 v2
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 parallel 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.
It can be shown from analytical calculations of line impedances that the mutual
impedances for positive and negative sequence are very small (< 1-2%) of the self
impedance and it is practice to neglect them.
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI
297
Application manual
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